Thermodynamic aspects of gas-phase electron attachment to transition metal tris (Beta-diketonate) complexes

thermodynamicasp00shar ( .pdf )

AA00002118_00001.pdf

THERMODYNAMIC ASPECTS OF GAS-PHASE ELECTRON
TO TRANSITION METAL TRIS(BETA-DIKETONATE)







By


PAUL SHARPE
-" a>


ATTACHMENT
COMPLEXES


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL
FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


UNIVERSITY


OF FLORIDA


1990
^- -.w

















ACKNOWLEDGMENTS


First


I must


thank my research director


Dr. David E.


Richardson


for his


understanding,


guidance


and support


throughout my graduate


career.


Dr. John R.


Eyler


and Dr.


Cliff Watson


also deserve


considerable


recognition


for their many


contributions


useful


discussions,


especially on


help with


the instrumentation.


No dissertation would be complete without


acknowledging


all the


friends


loved-ones


for their


support.


Foremost


in my mind


in this


regard


Stephanie Weinstock,


whose


love


understanding


have been


constant


source of


encouragement


over the


last


two years.


Also,


would


like


to thank my mother,


and my


late


father whom


know would


have been


proud.


Finally,


at the


will


University of


always

Florida


remember my


colleagues


especially Matt,


Casey,


in Graduate


Mike


School


T.M.


greatly


enhanced the enjoyment


of graduate


school


at Florida.


















TABLE OF CONTENTS



pa*e
ACKNOWLEDGMENTS........................................-.-...........-.ii


CHAPTERS


INTRODUCTION.


Overview of Research.
Introduction to Metal


Properties a
Description

EXPERIMENTAL


B-Diketonate C complexes.
G-Dn^ike'/^toateo Complexes.va


nd Applications of Metal
of the FTICR Technique..


PROCEDURES


B-Diketoi


. .. ..1
. .8
nates .......12
............. 18


AND RESULTS


Preparation of
Cnmle ets a


Tris(hexafluoroacetylacetonate)


omphjJa x ...... ............................~
Preparation of Tris(acetylacetonate) Complexes.
Preparation of Ruthenium Tris(B-Diketonates)...
Organic Compounds..............................
Electron Attachment Studies....................
Gas-Phase Spectrophotometry of Cr(hfac)3.......


. .........23
...........25
S .25
...........26
. ..... .. .26
S.......... .37


TERMINOLOGY AND CONVENTIONS
ION THERMOCHEMISTRY......


USED


Introduction.....................
Electron Affinities and Adiabatic
Potentials.....................
Stationary and Thermal Electron C


IN GAS-PHASE


... .. ... .

.... .... ..* ...


...........
Ionization

conventions.


.. ... 39
.. ...........40


INTRAMOLECULAR ENTROPY CHANGES FOR REDOX COUPLES
INVOLVING COMPLEX METAL IONS...........................48


Introduction.......


Statist
of Ga
Redox
Compari
Chang
Metal
The Rel
of Ga


ical


Mechanics Applied


s-Phase
Couple
son of
es for
Comple
ationsh
s-Phase


Intr
s Inv
Solut
Some
xes..
ip Be
Elec


to the Determination


molecular Entropy Changes for
olving Complex Metal Ions........
ion-Phase and Gas-Phase Entropy
Redox Couples Involving Octahedral


....
twee
tron


Mthfae1.- Comnleres...


. .51


.... ..-............................72
n The Free Energy and Enthalpy
Attachment to M(acac)3 and
- a a a a a a a a a a a a a a a -' 80


ABSTRACT......................... .... .. .......... .... *-*.*


.........................48










METAL-LIGAND BOND ENERGIES AND SALVATION ENERGIES
GAS-PHASE TRANSITION METAL TRIS(ACETYLACETONATE
COMPLEXES AND THEIR ANIONS.....................


)
...... ..


Introduction..............................................84
Electron Attachment Energy Relationships.................85
Homolytic and Heterolytic M-0 Bond Enthalpies in
M(acac)3(g) Complexes and M(acac)3"(g) Ions.............90
Relative Solvation Energies of M(acac)3(g)
and M(acac)3" (g)".*....................................99
Relative Solvation Energies of Ru(tfac)3(g) and
Ru(hfac)3 and Their Negative Ions......................108
Conclusions..............................................111


INTERPRETATION OF THE TRENDS ON THE ELECTRON ATTACHMENT
FREE ENERGIES OF THE TRIS(ACETYLACETONATE) AND
TRIS(HEXAFLUOROACETYLACETONATE) COMPLEXES OF THE
METALS Ti-Co USING THE SIMPLE LIGAND FIELD MODEL.......


Introduction.....................................
Thermochemical Relationships and Periodic Trends.
Conclusions......................................


........113
........114
S. ..128


REFERENCES.


BIOGRAPHICAL SKETCH..................................................
















Abstract


of Dissertation


of the University
Requirements


of Florida


Presented


to the Graduate


in Partial


for the Degree of


Doctor


Fulfillment


School
of the


of Philosophy


THERMODYNAMIC ASPECTS OF GAS-PHASE ELECTRON


TO TRANSITION


METAL


TRIS


(BETA-DIKETONATE)


ATTACHMENT
COMPLEXES


Paul S

August


harpe

1990


Chairman:


Major


David E


Department:

Estimations


Richardson,
Chemistry

of the free


Ph.D.


energies


gas-phase electron


attachment


to several


transition metal


tris(8


-diketonate)


complexes


at 350 K are


reported.


compounds


studied


are the tris(acetylacetonate)


complexes


(M(acac)3)


the tris(hexafluoroacetylacetonate)


complexes


M(hfac)3


of the


data


series


for the


of first


row transition metals


ruthenium complexes


Ru(acac)3,'


= Ga Co.


Ru(tfac)3


In addition,


and Ru(hfac)3


are


reported,


first


where


tfac


trifluoroacetylacetonate.


reliable estimations


of thermal


gas-phase


This work


electron


represents

attachment


energies


for a class


of coordination


compounds.


Electron


using


attachment


Fourier transform


free energies

ion cyclotron


for the


complexes


resonance mass


were obtained


spectrometry


(FTICR)


to monitor


charge-transfer


bracketing


and equilibrium


reactions


involving parent


negative


ions


trapped


in a mixture


of two gases.


gas mixture consisted of


a metal


complex


an organic


reference


compound,


for which


the electron


attachment


free


energy


is established


Theoretical


intramolecular


entropy


changes


some


redox


couples


involving


coordination


complex


ions


are estimated


and compared,


where

















electron


thermochemical


attachment


data


energy


energy
cycles


data are combined


that


lead


with


to estimations


other


of the


changes


in heterolytic M-O bond enthalpies


and solvation


free energies.


The observation


acetylacetonate


of charge-transfer


anions


bracketing


reactions


led to the determination of


involving

heterolytic


homolytic

Published


M-0 bond

estimatio


enthalpies

ns of the


for M(acac)3


absolute


neutrals


potential


of the


their al

standard


onions.

hydrogen


electrode


led to estimations


electron attachment


of the


to the complexes


free energy

in several


for solution-phase


solvents.


For


Ru(acac)3,


single


ion solvation


free energy


is estimated,


result


is discussed


comparison


to a


similar


estimation based on a


dielectric


continuum theoretical model.


results of


this


investigation


serve to


illustrate


relative


importance of


changes


in bond energies


and solvation


energies


that


determine


the magnitude of


redox


couples


involving


reduction


of tris(B-


diketonate)


complexes.

















CHAPTER 1
INTRODUCTION



Overview of Research


Many methods


have


been


used


for the determination


of gas-phase


electron attachment


energies


of atoms


and molecules.


These methods


be categorized


into theoretical,


semiempirical


and experimental,


these


various


approaches


have been


reviewed.


1 Although


some


of the


published data on


gas-phase electron


attachment


energies


has been


determined


from


solution-phase


studies


such


as polarography


and reaction


calorimetry,


the majority


experimental methods


that


of the

study


published work


gas-phase


negative


to date


stems


from the


ions directly.


These


methods were actively


developed


during the early


1970s.


Gas-phase


methods


rely


on a ready


source of


ions,


which


is by


far most


easily


accomplished by the


determinations


have


ionization of


been


performed


a gas.

on ions


Hence,


formed


the majority o

from volatile


precursors.


In the case of


metal-containing


compounds,


studies


negative


ions


have been


restricted


to those


formed


from metal


halides,


especially


are


metal


hexafluorides,


all gases or volatile


hexafluorides


have


oxyhalides,

liquids or


received


and carbonyls.


solids.


a great


deal


These


In particular,

of attention.


compounds

transition


It has


long


been


known


that


these


compounds


are the most


highly


oxidizing


compounds


known,


typically


having


electron


affinities


in the


range of


eV to 10 eV.


The majority


of experimental methods


involving


gas-phase


negative


inns


ths


t. l *IJVr- L ]i*si [ I i fl"( i


Is. ewa


k a a -


. a a s2


tn d~tarmi no


the ci orO-rnn


nt-I- nr.hrnonl-


can


4-












energy


an impinging


particle


that


causes


formation


or breakdown


of a negative


ion.


In the atom


impact


method,


beams of


energy


selected


neutral


alkali


atoms


are collided


with


a neutral


target


gas.


translational


energy


dependence of


the alkali


atom beam


is monitored


a function of


the relative


cross


section


for the


formation


of the


parent


anion


of the


target


molecules.


Using


this


method,


Compton


workers


have provided


estimates


for the


electron


affinities


of MoF6,


ReF6,


SeF6,


TeF6


and WF6.


A related method


involves


colliding negative


the negative

endothermic


ions


ions


into


is varied


electron-transfer


a target


gas.


and the threshold


reaction


translational


energy


for the


energy

onset o


is detected by the observation


formation


charge-transfer


transfer.


of product


or may


enthalpy


ions.


ion-molecule


be accompanied by


of formation


atom


of the


reaction may


transfer,


anion


such


of interest


be simple

as proton


can be


determined


from the enthalpies


of formation of


all other


species


involved


in the


reaction,


combined


with


the threshold


value


of the


translational


atomic


energy


and negative


of the negative


ion beams


ion beam.


in these methods


Although


can be


the energy of


controlled


within


a range of


energy of


0.1


, there


are


several


problems


encountered


prominent


in obtaining


are


distribution


accurate electron


sensitivity


of the


of the


translational


attachment


threshold


energies


energies.


energy to

the target


Most


thermal


gas,


weakness


of the


signal


around


the threshold


energy


lack


information on


initial


and final


states


of the neutral


molecules


product

results


ions.


Due to


for electron


the inherent


affinity values


problems


these methods


for transition metal


have


produced


hexafluorides


that


are


in considerable disagreement


For example,


range of


- 1 an~ -nfl


a
- CC 4 n 4-. 1 .. a


- a A~ a -2


A- ~ ~ ~* __ ^ -- _- A- ----A -


co-












the detachment


using photons


an electron


known


from a


energy


negative


generated by


ion,


according to


a laser,


or light


eq 1-1,

source


with monochromator.


+ hv


= AB +e


Photodetachment methods


use a variable


frequency


laser to detach


electron


from the negative


ion AB


threshold


energy


can be


obtained by using


a variety


of physical


methods


that


detect


either the


detached electrons or the


formation


of neutral


molecules


from the


anions


as a function


of photon


flux


and wavelength.


interaction


of the


negative


ions with


the photon


flux


has been


accomplished


in crossed


photon-molecular


beam experiments,


drift


tubes


and in ion traps.


In photoelectron spectroscopy


frequency


of the


photon


source


is fixed and


the energy


spectrum of


the emitted


electrons


is recorded.


For simple molecular


anions,


composed


a few atoms,


the energy


spectrum can be resolved

the detachment process.


into the vi

Determining


brational


transitions


the energy of


accompanying


the ejected


electrons


from the ground


vibrational


states


of the neutral


anion


leads


very


accurate determinations


of the electron


affinity


of molecules.


Using the photoelectron


spectroscopy method,


Lineberger


co-workers


have


determined


the electron


affinities of


several


carbonyl


complexes


Fe and Ni,


which are


in the


range of


0.6 2.4 eV.8'9


A limitation


extending the technique


to high


electron


affinity


compounds


such as


volatile metal


hexafluorides


is that


although


it may


poss


ible


identify


the energy transitions


in the spectra,


producing


a light


source


required wavelength


and flux


impractical


for compounds


such












Since


about


1982,


techniques


in mass


spectrometry that


are


capable


of following


the time


dependence of


ion-molecule


reactions


have


been


used


to determine


the electron


attachment


energies


of polyatomic


molecules.


In these methods


the equilibrium constant


for a gas-phase


charge-transfer


reaction


involving two


neutral


reactants


their


parent


negative


ions


is measured.


These methods


have


therefore been


described


as equilibrium methods.


techniques


in mass


spectrometry


that


have


been


used


for this


purpose


are pulsed


high


pressure mass


spectrometry


(PHPMS)1 0-15


and ion cyclotron


resonance mass


spectrometry


(ICR)


16-19


Electron


attachment


energies


for many


different


organic


compounds


have been


reported,


and values


are


in the


range


of 0.5 3


types


determinations


new


compounds


are reported each


studied


year.


is still


One


of the


expanding


advantages


and many more


of the


equilibrium method


over threshold methods


for determining


electron


attachment


energies


is that


the moleular


ions


formed after


ionization


are cooled


to the


same temperature


as the neutral molecules


from which


they


are


formed,


usually by


a thermalizing


bath


gas.


Electron


attachment


energies


are


therefore obtained


a definite


temperature


be combined with other


attachment


energies


thermochemical


in condensed-phases.


data,


especially


For example,


many


electron

of the


organic


compounds


studied


exhibit


reversible electrochemical


behavior


this


has led to estimates


of the


change


in solvation


energy


solution


phase reduction of


these compounds


14,15,20


A relatively


recent


technique


in mass


spectrometry that


also


capable of


monitoring the


time dependence of


ion-molecule


reactions


versatile


mass


spectrometry


powerful

(FTICR).


Fourier


transform


An important


ion cyclotron


difference


resonance


between FTICR


-A- ~-. -A.- -t. -~


. A












reactions


research


of metal


considers


containing


ion-molecule


compounds.


processes


21-43


Much


of the


involving metal


published

containing


ions


(mostly derived


from organometallic


precursors)


or bare metal


ions


(produced


by techniques


such


as laser


vaporization


of metal


targets)


The bulk of


this work has


centered


on the reactions


of bare metal


ions or


highly


coordinatively unsaturated metal


ions


with


bonds


21-34


Relatively


little


attention


has been


given,


however,


coordinatively


saturated metal


complexes


with non-carbon


donor


ligands,


such


as coordination


complexes


There were two principal


objectives of


the present


work.


first


was


to determine


the free energies of


thermal


electron


attachment


for a series


of coordination


complexes by using the


FTICR


technique,


thereby


extending the established


the determination


of free energies


charge-transfer

of electron at


equilibrium method


tachment


coordination


complexes.


The compounds


studied


in the


present


work were


tris(acetylacetonate)


(M(acac)3)


tris(hexafluoroacetylacetonate)


(M(hfac


complexes


complexes

Ru(acac)3,


of the series of metals M


Ru(hfac)3


and Ru(tfac)3


= Sc


were


- Co and


also


included


(where


tfac


= trifluoroacetylacetonate).


These


complexes


are


pseudo-octahedral


tris


chelate


coordination


complexes


in which


ligands bind


to metal


centers with


oxygen


atom donors.


Estimates


of the


free energies


electron


attachment


for these complexes


were obtained


in the


present


work,


this


represents


the first


reliable determination


of the gas-


phase electron attachment


energies


of coordination


complexes


under


thermal


conditions.


second


objective was


to determine


the changes


average


heterolytic metal-oxygen


(M-O)


bond


energies


that


occur


during


-phase


i












obtained


incorporating the electron


attachment


free


energy


data


complexes


into energy


cycles35


that


relate


the energy


for this


process


to the energy


for electron


attachment


to the


free metal


ions


the coordinated


heterolytic

attachment


bond


ions


in solution.


enthalpies


to the complex


To obtain


a value of


change


the entropy


change


average M-O

for electron


required.


Estimates


of entropies


could


be obtained,


in principle,


determining the


temperature dependence of


the equilibrium constants


the gas-phase


FTICR


instrument


charge transfer


used


in this


reactions.


There


study to determine


s no provision

the temperature


on the


dependence of


was


used


ion-molecule


to attempt


phase electron


reactions.


Therefore,


to provide estimates of


attachment


to coordination


entropy


statistical


changes


compounds.


mechanics


gas-


results


of the


calculations


reported


in this


study


have


provided


insights


into


magnitudes

involving


of entropy

coordination


changes


complexes,


for electron


both


attachment


in the gas-phase


processes


in solution


have enabled estimates


to be made


for the change


in heterolytic M-O


bond


energies.


M(H20)

represent


Data


and M(H20)


one of


of this


ions,


type


are scarce


where M are


series


first


complex


for metal


transition


ions


complexes.


series metals,


for which metal-ligand


bond


energies


and solvation


energies


are known


for both


ions


that


form


the redox


couple.


Generally,


even


less


is known


of the


thermodynamics


of redox


processes


at metal


centers


involving negative


ions.


data


obtained


for the


B-diketonate


complexes


in the


present


work


therefore


broaden


present


understanding


of the thermodynamics


of redox


processes


that


occur


at transition metal


centers


in different


coordination


environments.












enthalpy of

uncertainty


the enolic O-H bond


since


no experimental


in acetylacetone


data


introduces


are available.


From


the greatest


results


of thermal

presented


gas-phase cha

in the present


enolic O-H bond


enthalpy.


rge-transfer


work,

From


reactions


an improved

the original


involving


estimate i

1 reaction


aca


s made


c' ions,

for the


calorimetry


data


improved


estimates


are made


for the average M-0O


homolytic


heterolytic


bond


dissociation


enthalpies


for M(acac)3


complexes.


data,


when


combined with


the gas-phase electron attachment


energy


data


for the M(acac)3


complexes,


has allowed


the average heterolytic


homolytic bond


dissociation


enthalpies


for the


corresponding


gas-phase


M(acac)3


" ions


to be determined.


It has been


shown for M(H20


redox


couples of


first


transition metal


series


ions


that


the trend


in the magnitudes


reduction


potentials


for these couples


can be related


quite


successfully


to the


trend


in the electron


attachment


energies of


free M+3


ions


(the negative value of


the third


ionization


potential


of M(g))


correcting the reduction


potentials


for the difference


between


heterolytic M-0


bond


enthalpies


in the oxidized and


reduced


form of


each


redox


couple.


35-37


Although


the difference


in absolute magnitudes


electron

solution


attachment

can only b


energies

e accouted


between M+3

for by adc


ions


in the gas-phase


litionally


considering


solvation


energies


, nephelauxetic


effects


in the


completed


ions


and the


absolute


potential

variance


neglected


of the electrochemical


of the


when


sums of


compared


reference electrode


these quantities

to the difference


is generally


used,


small


in heterolytic


bond


periodic


enough


to be


energies.


trend


energies


in the difference between metal


between


the ions


that


form redox


-ligand

couples


heterolytic


bond


can be estimated


from


3+/2+












line


for the electron attachment


energies of


free


ions.


In this


way,


a simple explanation


is provided


for the


trends


in the


reduction


potentials.

the M(acac)3


The trend


complexes


in the gas-phase electron


in the


present


work


attachment


energies


explained by using


similar


approach


to that


taken


for the M(H20) 32


couples.


Introduction


to Metal


B-Diketonate Complexes


transition metal


belong to


great


many


the general

B-diketone


class


ligands


complexes

of metal


that


investigated


in the


B-diketonates.5,39


coordinate


present


There


to metals.


work


are


general


structure


of B-diketones


is shown


in Figure


1-1.


The most


common


ligands


hydrogen,


have R1


and R2


but several


= alkyl,


other


perfluoroalkyl


ligands


have been


and aryl

reported


groups,


which


and R3


R3 is


also


alkyl


or aryl.


?R2

0


Keto


Form


Enol


Forms


Figure


1-1.


Structure of


B-diketones


in keto and


enol


forms.


Figure


1-1 illustrates


the the keto-enol


tautomerism


that


exists


in many


B-diketones.


hydrogen


atom at


the B


ring


carbon


atom


S. ^^
\ R, R
C
0
0


\ /

























































Ligand


Abbrev.


Acetylacetonate


acac


Trifluoroacetylacetonate

Hexafluoroacetylacetonate


tfac

hfac












(cisoid)


conformation.


The proportion


of the enol


tautomers


generally


increases when


an electron withdrawing


group


such


as a halogen


atom


present


as R3.


The enolization


also


increases when


ligands


fluorinated or


contain


an aromatic


ring.


Substitution by


a bulky


group


(e.g.


alkyl)


at the


ring a or


carbon


atoms


causes


steric


hindrance


between R3


and RI


particularly


in the enol


tautomer,


this


together with

significantly


the inductive effects of


reduce


the proportion of


the alkyl

the enol


groups may


tautomer


at equilibrium.


Since


complexation


to a metal


is believed


to occur through


enol


form


ligand,


result


poor


attempts

yields.


to obtain metal


complexes


of these


ligands


often


complexes


transition metal


investigated


complexes


in the


of the ligands


present


hfac,


work are


tfac


tris-chelate


acac.


structures


structure


of the


of the


coordinated


complexes


ligands are given


themselves


are


shown


in Figure

in Figure


, which


shows


the two optical


isomers


that


exist


in tris


B-diketonate


complexes.


are












Table


1-1.


Structural


Details


of Metal


Tris


(B-Diketonatesj


Complex

V(acac)


V(acac)3(

Cr(acac)3

Mn(acac)3

Fe(acac),


Co(acac)3


Co(acac


(Bu4N*


Average


O-M-O
Angle

88.0


87.3

91.1


97.2


87.1


97.3


91.5


c)
salt)


acac)3


Cr(hfac)3

Fe(hfac)3


94.0

87.0

87.0


Bite


M-O


Average
Bond


Length/A


.979


.982

.952

.901

.992'

.898

.981


.000

.987

.999


aData

bData

CData

dData

eData

Data

Data

hData

'Data


taken

taken

taken

taken

taken

taken

taken

taken


from ref

from ref

from ref

from ref

from ref

from ref

from ref

from ref


taken from ref


3(a)a












For symmetrical


two optical


isomers


ligands


are possible


and R2

(Figure


in Figure


1-3).


are


The d


and 1


same),


optical


isomers


of Cr(hfac)3


have been


isolatated by


gas-chromatographic


techniques


by using


an optically


active


support.


For unsymmetrical


ligands


can exist


and R2


in a


cis or


Figure

trans


are not the


conformation.


same)


each


It has been


optical


found


isomer


using


gas-chromatograph equipped with


an electron


capture dectector


that


Cr(hfac)3

gas-phase.


unsymmetrical

investigations

intramolecular


undergoes


dynamic


cis-trans


Tris(B-diketonate)


ligands


have


intramolecular


transition metal


received


of the mechanisms


structural


that


isomerism


considerable


have been


isomerism


complexes

attention


in the


regarding


proposed


in tris-chelate


complexes.


50-52


several

X-ray


are


of the


crystal


tris-chelate metal


structures or


available.


"bite"


complexes


gas-phase

angle of t


studied


electron


ligands


in this


diffraction


and the M-0O


report,

structures


bond


lengths


obtained


from these


investigations


are presented


in Table


1-1.


"bite"


angle of


the oxygen donor


atoms


is in all


cases


is quite


close


, which


gives


a pseudo-octahedral


arrangement


about


central metal


atom of


0 donor


atoms.


Properties


and Applications of


Metal


B-Diketonates


In this


section


some


background


is given


of the


chemical


physical


properties


and applications


transition metal


B-diketonates.


The emphasis


on the M(acac)3,


M(tfac)3


and M(hfac)3


complexes


of the


first


study,


transition metal


although other


series,


complexes


which


are the


are included.


subject


Much


of the


of the


present

relevant


* ~1 nL a a a A.. t. .7 --A S S -


i i


u.^


_ *












Rather,


this


section deals


with


the more


interesting


relevant


miscellaneous


literature on


transition metal


B-diketonates


that may


serve


to acquaint

The physical


reader with


and chemical


these


properties


compounds.


transition metal


B-diketonates


have generated


a great


deal


of research


interest


since


they were

compounds

properties


first


stems


synthesized


not only


as coordination


in the


year


from their


complexes,


1887.


spectroscopic


but also


from


interest


in these


structural


their


remarkable


physical


properties.


Many metal


B-diketonates


are volatile,


which


Morgan


and Moss


in the


year


1914


to describe B-diketones


as the


ligands


that


"gave wings


to metals"


The factors


that


determine


volatility

octahedral


of metal


complexes,


B-diketonates


an increasing


have


been


amount


discussed.


of fluorination


Generally,


in the


ligand


leads


to greater volatility.


Hence,


for the complexes


studied


in the


present


work,


the order


of increasing volatility


is M(acac)3


< M(tfac)3


< M(hfac)3.


The M(tfac)3


complexes


are usually


only marginally more


volatile


than M(acac)3 due


to the dipole moments


present


in the


cis and


trans


forms


of M(tfac)3


complexes.


volatility


of transition metal


B-diketonates


has enabled


them


to be


applied


studied by


a variety of


to ligated metal


centers


physical


that


methods


exist


that


as ions.


are not readily


vapor-phase


He(I)


photoelectron


spectra


of M(hfac)3


and M(acac)3


complexes


have


been reported.


54,55


The spectra


were


interpreted


in terms


of elementary


molecular


orbital


theory,


which


yielded


information


concerning the


details


of the metal-ligand


bonding,


and in the


case


of transition metal


comply


exes


, information about


the the relative energies of


the metal


ligand


orbitals.












method


does


not require


high


vacuum and


accordingly


has the


advantage of


relative ease


for large


scale


application


with


poss


ibility of


coating


complicated


shapes.


Thin


films of


superconducting


YBa2Cu307


have


been


prepared


a process


that


involves


thermal


decomposition


flow of


a vapor mixture of


B-diketonate


precursors of


, Cu and


Ba in


argon.


There


are several


comply


exes


of yttrium and


copper that


sufficiently


volatile


and thermally


stable


to be used


for this


purpose


These


include


Y(dpm)3


Cu(acac)


and Cu(dpm)


". Barium


complexes


are


less


thermally


stable


experimental


and decomposition has


conditions.


The most


been


success


reported


this


under


respect


the

has been


obtained


with


Ba(fod)2,


where


fod = 2,2


dimethyl-6,6,7


7,8,8,8-


heptafluoro


octadionate.


B-diketonate precursors


volatilized

controlled

the gas mix


in separate


to give


ture before


sources


the desired


it reaches


and their


flow rates


stoichiometric


a high


ratio.


temperature


are


carefully


Oxygen

reactor


is added


containing


substrate onto which


the superconducting


layer


to be deposited


Water vapor


has been


added


to the oxygen


flow to


aid in the decompostion


of the complexes


hydrolysis.


Substrates


used


so far have


been


rTiO3,


Al203


and yttria


stabilized


zircona


(YSZ),


deposited


films


are


usually


5-10


pm thick.


After the decomposition


period,


variety


of annealing processes


have been


used


(depending


on the


B-diketonate


precursors


used)


to convert


initially


deposited


layer


into


superconducting


YBa2Cu307.


By this method,


films


of good


compositional


and dimensional


uniformity


are produced.


A similar method


has been


type


used


These


to produce

thin-film


thin


superconducting


superconductors


films of


have critical


TI-Ba-Ca-Cu-0


temperatures


range


of 90-120


K with


the onset


zero


resistance


at 65-100


-. S .t a


are


are


I a












The volatility


of metal


1-diketonates


has allowed


several


investigations


of their


gas-phase


positive


and negative


ions


using


mass


spectrometry


Much


of the work with


positive


ions


has concerned


the determination


appearance


potentials62


and mechanisms


fragmentation


subsequent


interesting work has


to electron


been reported by


impact

Pierce


ionization


Some


and co-workersM


investigation


of the secondary


ion mass


spectrometry


(SIMS)


laser


desorption


of solid


samples


transition metal


B-diketonates.


of the


aims of


the study was


compare


the ionic


species


formed by


conventional


El ionization


to to


those


formed


from


SIMS


and LD.


SIMS


spectra revealed


catonization of


intact


neutral M(acac)3


complexes


ionic


fragments


produced


in the plasma,


as well


as the


ionic


fragments


themselves.


Ions


masses


corresponding to the


following


stoichiometries were observed M(acac) +


, M(acac)2


, M2(acac)3


M2 (acac) +. These


species


had also


been


observed


in a


study


of M(acac)3


complexes


using


high


pressure mass


spectrometry.65


Catonization


neutral

chloride


M(acac)3


of these


, Ag


cations


and NH4+


was also


were mixed with


found 1

Solid


to occur when


sample of


metal


diketonate.


SIMS


spectra


of mixed


samples


of M(acac)3


complexes


of two


different metals produced mixed metal


clusters


of the


same general


formula.


When


certain mixtures of


a metal


B-diketonate,


a chloride


different


transition metal


and a cationizing


agent


were vaporized,


ligand


exchange was


and NH4C1


found


produced


to occur.

Cr(acac)*


For example,


, Cr(acac)2


a mixture


SFe(acac) +


of Fe(acac)3,


CrCl3


, Fe(acac)2


acac)-CH3 ]


For other mixtures


no ligand


exchange


fragments


were


detected.


spectra


of laser


desorbed


samples


produced many


of the


same


fragment


ions


observed


in the


SIMS


experiments.


Interestingly,


rn a rr-nrao ni tho nroaoni -hfl1r-~ 1 4nnnd ovr'hnnrwo Ii a boon nhantvnd


* ---1-


S
nfl


1 < nan


h hoon


rnaoi~xror


&


LJI i r ar _


f-


T n












exchange was


found not


to occur.


To probe the


structure of


bimetallic


clusters,


Pierce


and co-workers


used


collisional


dissociation


to observe


pathways


for fragmentation.


The resulting


spectra


indicated


that


the cluster


ions


could


not be considered


simply


as metal


cations.


Rather,


a stable


structure


involving metal


atoms


was


invoked


with


possible metal-metal


bonding.


Reports


on the negative


ion mass


spectra


of metal


B-diketonate


complexes

volatility


have


focused


very


largely


large


cross


on M(hfac)

sections


complexes


for electron


due to their


high


capture.


thrust


of the work has


been


to determine


the fragmentation


pathways


the parent


ions


following


70 eV ElI


ionization.


66-71


possibility


using negative


ion mass


spectra


some metal


B-diketonates


as an


analytical


technique


in the field of


ultra


trace metal


analysis


has been


investigated.67


Some results of


previous


investigations


of negative


ions


formed


from metal


B-diketonate


precursors


are discussed


in the


experimental


section


this


dissertation,


in comparison


to the


results


obtained


in this


work.


A large number of


metal


tris(B-diketonate


complexes exhibit


reversible electrochemical


behavior,


especially those containing the


metals


Ru and


There


are numerous


reports


on the


effect


of the


ligand R


substituents


(Figure


1-2)


on experimentally


observed E1/2


values


for electrochemical


reduction


of these complexes


72-76.159-164


substituent


effects


are quite


pronounced.


For example,


values


reported


for Ru(dpm)


and Ru(hfac)3 differ


1.84


V in


dimethylformamide.


For series


of tris(B-diketonate)


complexes


of the


same metal,


the trends


in reduction


potentials


correlate


predictably


with


the electron


releasing or withdrawing


nature of


ligand


ring


aithal- 4 Fnort4- a 4-ho nrAor V.,; OOO .aaA..n. 4 an Ca.. n am 1 a.. an a LItt


- -


LJ


f~hl l o


*/-/4 /^+ /^.*"












aromatic"


character


It has been


shown


that


there


a strong


correlation between


the trends


values


for the reduction


series


of tris(B-diketonate)


complexes


of the same metal


and Hammett


parameters


that


have been


derived


from observations


of the effect


ring


substituents


on the


thermodynamics


and kinetics


involving


reactions


of organic


aromatic


compounds.


Interestingly,


for complexes


varying


with R3


= H (Figure


1-2),


there


is generally


a closer


correlation between El/2 potentials


a para


parameters


than meta


parameters


despite


the meta


position,


with respect


to the metal


center,


of the carbon atom that bears


the ring


substituent.


However,


since


oxygen


donor


atoms


in the


ligand


are para


to the


substituted


ring


carbon


atoms,


the phenomenon has


been attributed


to the distribution


electron density


values


at the oxygen


can be explained


atoms.


in terms


From this


of ligand


standpoint


field


value of


theory


considering the varying magnitude of


the spherical


component


of the


ligand


field


The

manifest


produced by the oxygen


quasi-aromatic


nature of


by the occurence of


donor


atoms.


coordinated B-diketonate


electrophillic


substitution


ligands


reactions


metal


B-diketonate complexes.


These


reactions


produce


complexes


that


are not


easily


formed by normal


reaction


routes.


Substitution


occurs


B carbon


electrophiles.

halogenation,

formylation.


atom of


The wide


nitration,


The reaction


ligand

variety


(Figure


1-1)


of reactions


diazotization,


conditions must


with


a variety


can be classified


thiocyanation,


be chosen


into


acetylation and


so that


acid


labile


B-diketonate


rings


are not


degraded.


The most


widely


studied


complexes


are therefore


ruthenium(III)


which are


those of


chromium(III)


not hydrolyzed


in acid


cobalt(III)

solution.


and R2












Description


of the FTICR


Technique


Production,


traopinQ,


and mass


analysis of


ions


In 1974


Marshall


and Comisarow77


developed


a method


of applying


Fourier transform technique


to the analysis


of the masses


relative


abundances of


ions


trapped


an ion cyclotron


cell.


simultaneous


detection of


many


ions


over


a wide mass


range


circumvented


many

mass


of the


limitations of


spectrometry


ICR)


the original


technique.


Since


scanning


then,


ion cyclotron resonance


Fourier


transform


method,


known


as Fourier transform


ion cyclotron


resonance mass


spectrometry


(FTICR)


has developed


into


a powerful


versatile


technique


in mass


spectrometry.


FTICR


technique


is based


on the


classical motion


of ions


described by elementary


laws


of electromagnetism.


The magnetic


force


(Lorentz


force,


= q(VxB)


acting


on a particle of


mass


M, charge q,


initial


velocity


V in


a field of magnetic


induction


B causes


follow


a helical


path


Figure


1-4.


The constrained


circular motion has


frequency

frequency


in Hz given by v


falls


in the


= qB/2irM.


range of


This


radio wave


frequency


is the cyclotron


frequencies


(0.01


- 2.00


MHz)


for magnetic


fields


on the order


of 1


tesla.


To prevent


ions


from


travelling


along the


helical


path


and being


lost


, ions


are


produced


between


two trapping


plates perpendicular


to the magnetic


field.


These


plates


are maintained


at a repulsive


potential


(typically


or -1 volt


for positive and negative


held


in a


defined


ions,


region between


respectively), a

the two plates.


ions


thereby


Excitation


sets


of plates


magnetic


and detection of


(transmit


field between


and receive


trapped

plates)


ions


lying

n tho


require


along the


additional


axis


of the


the tranoino nlateB.


are


heavl




proT



















electron
collector


receiver
plate


trapping
plate


transmitter


receiver


grplate


grid


filament


tra pping
plate












used


in the


present


study


has a 2


tesla


superconducting magnet).


Ions


can be


formed


the cell


from the


low background


pressure


admitted


sample by


an ioni


zing electron


beam


passing


through


small


holes


in the


trapping plates or


by photoioni


zation


via irradiation


through


semi-transparent


grids


one or more


plates


. Application


external


oscillating


electric


field


across


transmit


plates


at the


characteristic


cyclotron


frequency


an ion


causes


ions


of that


mass


cell


orbits of


to move


larger


into resonance with


radius.


The kinetic


the applied

energy of


field


spiral


ion is given by


= 27r2


into


2 2


where


resonance with


r is the radius


the applied


electric


of the orbit.


field


As the


their motion


ions move

is shifted


from having


a random distribution


simultaneously moving


phase with


of phases


to that


the applied


field


of all


as a "packet"


ions.


If the


applied


field


is turned


or moves


out of phase with


ions,


ion packet


persists


long enough


to induce


an image


current


the detect


plates80 before


collisions with


neutral


molecules


restore the


initial


cyclotron


random distribution


frequency


of phases


of the ion packet


induced


contains


image


information


current


in the


at the


time


domain


about


the frequency


(mass)


of the


ion,


inten


sity of


signal


produced


is dependent


on the


ion population.


In order to


simultaneously detect


the masses


populations


many


different


ions


present


in the cell


, a fast


radio


frequency


sweep


applied


to the transmit


plates


corresponding to


the mass


range of


interest.


As each


ion of


a particular mass


moves


into


resonance


superposition


signal

stored


of image


is amplified


a computer.


currents


digitized b

The rapid


is generated

y an analog


sweep/detect


in the detect


to digital


circuit.


converter


is repeated many times












a mass


spectrum.


magnitude of


The high mass


the magnetic


field


range

, with


ais determined


increasing


primarily


resolution


by the


toward


lower


masses.


tesla


field


yields good mass


resolution


up to approximately


3000


amu.


Thus,


FTICR technique


has the


high


resolution


at large


values


complexes.


required


to study many


The lower mass


limit


higher molecular weight metal

is governed by the maximum rate of


signal


digitization.


With


a 5.2 MHz digitizer


and a 3


tesla magnet,


this


limits


the detectable masses


to >17


amu.


A lower magnetic


field


allows


the detection of


accompanying decrease


important

high mass


lower mass


ions


such


as OH"


with


resolution.


Manipulations


of ions in the trao


Between


the ionization


and detection


events


any one


ionic


mass


be kinetically


excited by


application


a single


frequency pulse


transmit


plates.


A range


of masses


can be excited by


a frequency


sweep.

absorb


Selected


sufficient


ions


can be ejected


energy


to spiral


from


the cell


out to orbits


completely


of such


large


if they

radius


that


they


strike


the cell


plates


ion ejection).


a low amplitude


pulse or


sweep


is applied,


the kinetic


energy


ions


can be


increased without


ejecting them


from the cell


technique


can be


used


to explore endothermic


of reactants,


and this


reaction


translational


channels by

excitation


increasing the


one way


energy


by which


structural


and thermodynamic


information


can be obtained.


An important


factor


contributing to the great


versatility of


FTICR


is that


tailored


pulse


sequences


can be


applied


in almost


combination.


can

















CHAPTER


EXPERIMENTAL PROCEDURES


AND


RESULTS


Preparation of Tris(hexafluoroacetylacetonate)


Scandium and


Complexes


Qallium tris(hexafluoroacetvlacetonate)


aqueous


chloride,


solution


containing


an excess of


ammonia


approximately


solution


was


gram of

added,


scandium or


which


gallium


precipitated


Sc(OH)3 or Ga(OH)3


respectively.


The precipitate was


filtered,


washed


and dried and


then


refluxed


one hour with


a 3-fold molar


excess


hexafluoroacetylacetone


(20%


in light


petroleum ether).


When


cool,


reaction mixture was


filtered,


and the


filtrate evaporated


to yield


colorless


crystals


(Sc(hfac)3)


or pale orange


crystals


(Ga(hfac)3


crystals were

purification.


sublimed at


torr


and 40-50C to effect


further


Titanium and


vanadium tris(hexafluoroacetvlacetonatei.


Both


these


complexes

standard

manifold.


light

TiCl3


are air


techniques


sensitive


involving


A 3-fold molar


petroleum ether)


a Schlenk


tube


was


and preparation


Schlenk tubes


excess


added


against


was


achieved by using


and a Schlenk argon/vacuum


of hexafluoroacetylacetone


to approximately


a flow of


argon.


gram of


(20%


VCl3 or


The mixture was


refluxed


for three


hours


under


a blanket


argon


then


allowed


cool.


solvent


containing the


dissolved


product


was


decanted


from


any unreacted


solids


into


a second Schlenk


tube,


which


had been


purged


with


argon,


prior to the


transference


using


a cannula


with


filter


attachment.


solvent


was


removed


by vacuum


to yield


chocolate












Chromium


tris(hexafluoroacetylacetonate).


This


compound


available


commercially


from Strem Chemicals


Ltd,


was


used


received.


ManManese trislhexafluoroacetvlacetonate).


The most


convenient


and simple method


of preparation


for this


complex was


found


to be that


reported by


Evans


and co-workers.55


Approximately


gram of


Mn203


added


to a


Schlenk tube,


followed by


a 3-fold molar excess


hexafluoroacetylacetone


(20%


in light


petroleum ether).


The mixture was


refluxed


for 48 hours


under


an argon


atmosphere


then


allowed


cool.


The resulting


black


solution


was


filtered


concentrated


yield


dark


green


crystals,


which


were


purified by vacuum


sublimation.


Only moderate


yields


of Mn(hfac)3


are obtained by this method,


since


it is


simple


for the gas-phase


and convenient,


studies


and only milligram


reported


amounts were


in this dissertation,


required

procedure


adequate.


Cobalt


tris(hexafluoroacetvlacetonate).


The most


convenient


method


for the


preparation


of Co(hfac)3


was also


found


to be that


reported by


Evans.


To approximately


gram of


cobalt


trifluoride


(CoF3)

which


in a Schlenk tube was


in this


reaction acts


added


gram of


as a hydrogen


anhydrous


fluoride


sodium


scavenger.


fluoride,

A 6-fold


molar


excess


(to the


amount


of CoF3)


of cooled


hexafluoroacetylacetone


added slowly to


powder mixture and


then


reaction mixture was


refluxed


one


hour.


Note that


no solvent


is added


to the


reaction


mixture.


During the


reflux period


the solution


turned


deep green.


Approximately


20 cm3


of light


petroleum ether was


then


added


to the


reaction mixture


, which was


stirred


and then


filtered.


solution


concentrated


to yield


dark green


crystals


of Co(hfac)3,


which


were


was


was


was


was












Preparation of


Tris(acetvlacetonate)


Complexes


except


11 the M(acac)3

for Ti(acac)3.


sublimation.


complexes


were


purchased


The compounds were


The Ti(acac)3


complex,


it is necessary to exclude


like


atmospheric


(Strem


purified

Ti(hfac)3


oxygen


Chemicals


before


is air


from


use


Ltd.)


vacuum


sensitive,

reaction


mixture during preparation by using


Schlenk apparatus


as was


done


V(hfac


Ti(hfac)3.


The complex was


prepared by


slowly


adding


mixture of

solution o


grams


of acetylacetone


f approximately


gram of


and 2

TiCl3


grams of

stirring


triethylamine


in 25 cm3


to a


of ethanol,


under an

addition


argon


atmosphere


and refluxing


accompanied by


reaction mixture


is not


a dark blue


necessary.


coloration


Formation


in the


becomes


of the


solution.


hot during the


complex


After


stirring


for 1


hour the


solution


containing the dissolved


product


was


transferred


to a second Schlenk tube


that


had been


purged with


argon.


solvent


removed by vacuum to


yield


dark blue crystals of


Ti(acac)3.


Purification was


ethanol/water mixtures

sublimation before use


Preparation


effected by repeated recrystallization


The product


was


of Ruthenium


from degassed


further purified by vacuum


Tris(B-diketonates)


ruthenium B-diketonate


complexes


investigated


in the


present


work


are


Ru(hfac)3,


Ru(tfac)3


and Ru(acac)3


complex


hfac)3


available


from Strem Chemicals


Ltd and was


used


as received.


The other


two complexes

reported by E


were


ndo


prepared by


and co-workers.


using the


rutheniumm blue"


Approximately


grams


method

of hydrated


-7


was












this


time


initial


orange


color of


solution


became


almost


black.


A 9-fold molar


excess


of ligand


(trifluoroacetylacetone or


acetylacetone)


was


added


to the


reaction mixture which


was


then


allowed


to continue


refluxing


for an additional


hour,


during which


time


solution became


red.


Next,


12 grams of


potassium hydrogen


carbonate


dissolved


in 50 cm"3


de-ionized water was


added


to the


flask


dropwise


over


a period


of 10 hours


while the


reaction mixture was


continued


to be


refluxed.


flask was


cooled


and the


solvent


was


evaporated by using


a rotary evaporator.


Benzene was


added


to the


flask


to dissolve


residue,


which


was


then


washed


with


three


portions


of 1 M


sodium hydroxide


solution.


The washed benzene


solution


was


dried


standing


over


purified by


anhydrous


sodium


loading onto a


1/2"


sulfate.


Finally,


chromatography


column


product

packed


was


with


mesh


alumina.


The column


was eluted with benzene


resulting


solution

crystals


concentrated

(Ru(tfac)3).


to yield or

No further


ange crystals

purification


(Ru(acac)3)


or red/orange


of these compounds


was


found


to be necessary.


OrQanic


Compounds


The organic


compounds


employed


in the


present


study were


purchased


from


commercial


sources


and used


without


further


purification.


extraneous or


fragment


ions were detected


in their


negative


ion mass


spectra.


Electron Attachment


Studies


J i D L m












dependence of


populations


of parent


negative


ions


formed


from a


mixture of


known


partial


pressures


of two


reactants


are monitored


they


charge


transfer with


the neutrals.


For the reactions


indicated


2-1,


free energy


involved


for electron


capture


species


be bracketed


within


the lower


limit


of the


known


value


for A and


upper


limit


of the


known


value


+ B


+ C


+ B"


+ C


When


the free energy


change


is small


kcal


with FTICR)


as in the case


, the equilibrium populations


the ions


can be measured.


+ B"


neutral


reactants


are in large


excess


and their partial


pressures


not vary during the


reaction


The equilibrium constant


for the


reaction


in equation 2-2


can be obtained


from the ratio of


the equilibrium


population


reactant


spectrometer


calibrated


of the two ions,


gasses.


and the ratio of


Measurement


is achieved by using


for each reactant


the partial


pressures


an ion


by using


gauge.


an external


pressures


on the mass


gauge was


baratron


capacitance manometer


in the


pressure


range of


torr.


- 10-5


Special


pressure


calibration


procedures


developed


for the FTICR


systems were


used


that


ensure


uniform reactant


gas pressure throughout


system by


adjusting the


relative


pumping


rates of


the two diffusion


pumps


connected


to the


high


vacuum


chamber.83


The equilibrium


constant


a a a S S


can


I I


m


A


*












charge-transfer


equilibrium method has


been


used


in ICR


experiments16-19


and PHPMS experiments10d15


to provide electron


attachment


energies

organic

enthalpy


for a large

compounds st


change


number


died,


(AHrxno)


of organic


compounds.


the corresponding


have been obtained by


entropy


For many of


change


following the


temperature


dependence of


the equilibrium.


11,12,14,15


The results


have


produced


ladders


of multiple overlapping values


AGrxn'


AHrxn


and AS


pairs


of organic


reactants


such as


substituted benzophenones,


nitrobenzenes,


10,11,13,17-19


quinones14,19


and dicarbonyls.


absolute


values


for electron


capture


by each


compound,


(defined by


AGa


for the


reaction A


= A')


are obtained by


including


an external


standard


in the


ladders


for which AHa


and AS


are well


established.


For example


EA of


SO2 has


been accurately determined


to be 1.097


0.036 eVM


and 1.107


photoelectron


0.0008 eV85


spectroscopy of


in two

SO2",


independent


ind SO?


investigations


is the reference


compound


chosen


in the EA


investigations


of Kebarle.


value


evaluated by the methods of


statistical mechanics


from


structural


and spectroscopic


data.


Electron attachment


and electron


transfer


ecuilibrium


studies


using the


Nicolet


FT/MS


1000.


Gas-phase


charge-transfer


reactions


type outlined


2-1 and in


were


studied


in the


present


work by using


a Nicolet


FT/MS


1000


Fourier


transform


ion cyclotron


resonance mass


spectrometer


(FTICR


A diagram of


instrument


used


given


in Figure


2-1.


The technique used


in the


present


work was


similar to


that


reported


previously


in ion cyclotron resonance mass


spectrometry


(ICR)


investigations.


and pulsed high

e temperature of


pressure


mass


the reaction


spectrometry


cell


(PHPMS)


was measured


under


was



































Ion Gauge


Superconducting Magnet
II

SSorxjs Prb
* I


Inlet System


-. -- ..-.-..--- .-.--.-..- ..


Ion Trap

Gate Valve Baratron




Mechanical Pumps


Diffusion Pump


Inlet Diffusion Pump












measured


equilibrium constant,


and a value of


for the


organic


compound

compound

organic

to admit


at the reaction


from the


compounds


tabulated


temperature


values of


and the M(hfac)3


into the mass


spectrometer


of 350 K (obtained


and AS ).


complexes


through


were


for each


Most


organic


of the


sufficiently volatile


leak valves


without


heating.


Less


volatile organic


and the M(acac)3


complexes


were


sublimed


tip of


a solids


temperature of


probe


- 350 K.


placed well


Negative


away
ions


from the


were


ion trap,


produced


which


was


from neutrals


FTICR


capture by


trap by


the metal


capture of


low energy


complexes was,


electrons


in most


cases,


Electron


accompanied


varying


amounts


of fragmentation.


Parent


ions were


selected


from


these


fragments


ion ejection


techniques.


To approach


collisional


thermalization


ions


prior


to the


ion/molecule


reaction,


FTICR relies on a


set thermalization


period


between


pressures


ionization


in this


and detection


study were


of product


in the 10"6 torr


ions.


range,


Typical


reaction


but a bath


such


as argon


or cyclohexane


can be


added


to reactant mixtures


if lower


reactant


pressures


are used.


For both bracketing


and equilibrium


experiments


a thermalization


period


of 1


s was


used


Assuming


a second


order


collision rate constant


pressure of


10o6 torr,


each


molecule"1


ion collides


sec


an average of


about


at a total

it 30 times


with


neutral


reactant


molecules


before


charge-transfer


reactions


were


followed.

populations


When


a charge-transfer equilibrium was observed,


were determined by measuring the


relative


abundance of


parent


ions over


suitable


time


intervals


until


they


reached


constant


value.


The equilibration


could


be followed


for long


reaction


times


" 20


s) ensuring


complete


thermalization.


At the


reactant












equilibrium.


The electron


attachment


energies


for all the


compounds


studied


are presented


in Table


2-1.


results


are also presented


Figure

in the


2-2 to illustrate


present


the organic


reference compounds


that


were


used


work.


Table
for M(


2-1.


acac)


Free


Energies


of Electron Attachment


3, M(tfac)3 and M(hfac)3


Complexes.


(kcal


mol'-)


at 350 K


Sc(hfac)3


hfac)3


V(hfac)3

Cr(hfac)3

Mn(hfac)3

Fe(hfac)3

Co(hfac)3

Ga(hfac)3

Ru(hfac)3

Ru(tfac),


-64 3c


-69 3c

-73 2b


Sc(acac)3

Ti(acac)3

V(acac)3

Cr(acac),


-67 3c


-109


-93)


(-97)a

-60.4

(-89)a

-64.0 b


Mn(acac)


a Fe(acac)


Co(acac)


0.5b


-24.9


-20 1c

-59 3c


-43.0


0.5b


-47 2c


Ga(acac)


Ru(acac)


-38.7


0.5b


t 0.5b


aEstimated


value


values


for M(acac)3


obtained by


complex


adding


50 kcal


mol-1


to corresponding


(see text


bValue obtained from
Reference compounds


measured equilibrium constant.


given


in Figure


CValue


obtained by


bracketing


(see eq


2-1).












.--Mn(hfac)3 (109)


-*Co(hfac)3 (97)
Fe(hfac)3 (93)

Ru(hfac)3 (89)


Chlorine atom 83.4


CN
C
CN


V(hfac)3


73 4


Ti(hfac)3 69


Ru(tfac) 64.0


Cr(hfac)3 67

Sc(hfac)3 64


F F


Ga(hfac)3 60.1
Mn(acac)3 59


-56.0-


Co(acac)3 47


- 45.3


Fe(acac)3 43.0


NO2-6 CN -- 38


.8 -


26.3-


Ru(acac)3 38.7


V(acac)3


- 21.3--


Cr(acac


CH3 CH3


19.5-


48.7- -


42.7 --


CN
= CS












Electron


attachment


enerav


acac"


radical.


electron


attachment


energy


of acetylacetonate


radical


was


determined by the


bracketing method,


in which


occurrence or non-occurrence


of charge


transfer


reactions


involving


acac


- ions


with


organic


reference


compounds


were observed


2-1).


acac


" ions were generated by


heating


Co(acac)3

pressure


tip of


(-10"7


torr)


a solids


of Co(acac)3


probe


in the


to produce


FTICR main


a low partial


chamber.


Acetylacetonate


anions


were


produced


following


electron


impact


ionization


of the gas.


following the


time dependence of


population


acac


- ions


in the


presence of


approximately


torr


each


a series


of organic


reference compounds,


it was determined


that


acac


charge-transferred to


2,6-dichlorobenzoquinone,


tetrafluorobenzoquinone,


which


sets


limits


of the electron


attachment


energy


at 59 3


kcal mol"


(see


Figure


2-2).


Consistency


of Electron Attachment


Energy Determinations.


Although


lower


operating pressures


of ICR and FTICR,


compared


to PHPMS,


enable


low volatility


compounds


to be studied,6


this


also


introduces


greater uncertainty


in the measurement


of reactant


pressures.


To check


consistency


of the


results obtained


in the


present


work with


those


of previous determinations,


AGrxn


for the reaction


= 1,4-


dicyanoben


zene;


= 3-fluoronitrobenzene was measured.


For this


reaction


at 423 K Kebarle


has found


AGmrxn


= -3.2


kcal


cal mol


FTICR at


giving


a value


350 K we obtained


AGrxn


of AHrxn'

= -2.8


of -2.1 kcal mol"1


kcal


mol-1


which


In the


together with


the previous


determined entropy


change gives


AHrxn = -1.9 kcal


The discrepancy

temperature of


of 0.2 kcal


mol


the neutral


probably


error


arises


from


in measuring


uncertain


equilibrium


ann n A. -a J-I--~ --a -, --- A--- A- ~- 1 2.....2A. -r -3-,-~-- A a- -


not


mol


A


I i L


A-, 1l-












estimate of


the expected error


for free


energies


determined


equilibrium reactions,


and conservative


uncertainties


of 0.5 kcal


mol-1


assigned


to values


of AG
a


determined by equilibrium to


account


experimental

uncertainties


uncertainties,


in the assigned


including


temperature,


pressure


thermodynamic quantities


for most


of the


reference


compounds.


Electron


attachment


to tris (hexafluoroacetvlacetonate)


complexes


The M(hfac


complexes


studied


in this


investigation


were


those of


first


row transition metals


from Sc-Co,


Ga and Ru.


These


complexes


particularly volatile


and are easily


admitted


into


FTICR through


leak


valves on


inlet


system.


It has been


shown


previously70',71


that


for a series of

fragmentation f


first


row transition metal


following electron


capture


M(hfac)3


increases


complexes


from


that


left-to-right


row.


same general


trend


was observed


in the


FTICR


in this


work.


The major pathway to


fragmentation


was loss of


a ligand


ion,


this


ion predominated


in the mass


spectra


Fe and Co complexes


immediately


after


electron


capture.


A few hundred milliseconds


after


ionization,


the parent


was formed by


charge


transfer


to the


neutral


complex


from the


fragment


ions.


After


a suitable


period


time,


any remaining

By observing


fragment


ions


charge-transfer


were ejected


reactions


from the


cell.


involving M(hfac)3


complexes


organic


reference


compounds,


was


found


that


few had


-AG values


complexes

reported.

Fe(hfac)3,


that


as high

d values


as that


of the complexes.


greater than


Although AG a values


Co(hfac)3


the order of


any of the organic


could


and Mn(hfac)3

a AG values


Co and Mn


compounds


so far


not be experimentally measured


estimates


runs


parallel


were obtained by noting


to the


series


are


are












was observed


which


of the two parent


negative


ions


predominated


after


a charge

complexes


transfer period.

was determined.


relative order


The difference


for the


in AG values


series


between


V(acac)3

results


and V(hfac)3


of two


was determined


to be


-50 kcal


separate equilibrium reactions.


mol"'1


Assuming


from the


a constant


difference of


50 kcal mol1


the other metals


between

series,


the M(acac)3

estimates coi


and M(hfac)3


uld be made


complexes


for the


M(hfac)3


complexes


= Ru, Fe, Co, Mn)


since


those


for the M(acac)3


complexes of

in this way


same metals were measurable.


have been


in parentheses


Values


of AG


obtained


in Table


substance with


chlorine


atom,


the highest


Cl-(g)


accurately


was included


known


electron


in the


study


affinity

of charge


transfer


reactions


with


the metal


complexes.


Electron


capture by


background


pressure of


Fe(hfac)3


with a


small


partial


pressure


of benzyl


chloride


produced


complex.


was


Cl (g

found


in addition


that


when all


to the ions


ions


except


formed


from


chloride were


the metal


ejected


from the cell


and its subsequent


reaction


with Fe(hfac)3


was


followed,


chloride


ion regenerated Fe(hfac)3


" by


charge transfer,


indicating that


the electron


attachment


energy


of Fe(hfac)3


> 83.4


kcal mol"1


accord


with


value estimated


above.


Charge


transfer occurred


from tetrachlorobenzoquinone


(C14BQ)


Cr(hfac)3,


an equilibrium reaction


was


not observed


in the


reaction


with


Sc(hfac)3


as the reaction


was


hampered


rapid


formation


of adduct


ions


(hfac)3.Cl4BQ]


Sc(hfac)3]2


Electron


attachment


to tris(acetylacetonate)


complexes.


M(acac)3


complexes


has been


were


studied


previous


for the series


noted


that


of metals


cross-section


Sc-Co,


Ga and Ru.


for electron


capture by


U


$4 ref ar., 4- -a ^ ne 4 4- 4 an mns4- n 1


r'^


^" ^IaVTi


I












substituents


in the


former.


The same general


effect


was


observed


this


report


for the complexes of


the metals


Cr to Co


The only


produced


from


ionization


of the


neutral


with


the electron beam was


ligand

charge


anion,


transfer


but unlike


the M(hfac)3


to the neutral


complexes


complex to


form


, the


ligand


the parent


ion did


-on.


Parent


negative


ions


of these complexes


could only


be obtained


in reasonable


yields

electro


following ch

n attachment


emical

energy


ionization by an

. In performing


organic


compound


experiments


with


of lower

these


compounds,

of ligand

detectable


therefore,


anion


from


fragment


was necessary to


the cell.


ions


The Ti


and had large


eject


relatively


and V complexes


cross


sections,


large


amounts


produced


in accord with


trends


in stability of


the ions


noted above.


The difference


in the electron


withdrawing


effect


between


in the


series


of complexes was


also observed


to markedly


reduce


values


of AG


for the M(acac)3


series


relative


to the M(hfac)3


series,


and the values


of AG


fall


well


within


range of


those


the organic


compounds


in the reported


electron


transfer


free energy


ladder,


which


extends


from


approximately


10-75 kcal mol1


This


enabled


bracketing


and equilibrium reactions


2-1 and


2-2 to be


followed


for the entire


series


of M(acac)3


complexes.


The Cr(acac)3


ion, although


initially produced


in the FTICR


cell,


was


unstable


underwent


rapid


loss of


ligand


a rate


that


increased with


total


pressure of


dissociation.

previously. 8


the system,

instability


Bracketing this


indicating


a collisionally


of the Cr(acac)3


compound


through


has been


induced

observed


charge-transfer


reactions


was therefore hampered by


competitive


ligand


loss,


produce


greater uncertainty


in the


result.


Parent


negative


ions


could












A value of


In contrast


for the


to all the other


Ti(acac)3

complexes


complex

studied,


could


not be obtained.


Ti(acac)3


or its anion


did not


undergo detectable


electron


exchange


in the


time


scale


obtainable with


the FTICR,


even


with


relatively


high


pressures


neutral


gas.


Exothermic


charge


transfer


reactions


involving


Ti(acac)3


with

10"4)


various c

over the


)rganic rE

range of


sactants were too


to 1


slow to


eV of driving


follow


force


(krxn/kco


llision


The cause of


this


unexpectedly


slow ga


s-phase


charge transfer


not known


and would not


have


been


predicted


for a dl/d2


redox


process.


Charge-transfer


equilibria


were observed


for the


V and


complexes


, and results


for Cr


Mn and Co were obtained by the


bracketing


technique outlined


Gas-Phase


Spectroohotometrv


of Cr(hfac)3


gas-phase


visible


spectrum of


Cr(hfac)3


was


determined


order


to compare the


spectrum to


that


of Cr(hfac)3


in solution


(see


chapter

designed


The ga


sample


cell


s-phase


with


spectrum was


10 cm path


obtained by using


length


and fitted


with


a specially


heated


quartz


windows


a temperature


and separately


a few degrees


heated


cell


cooler than


body.

that


The body was maintained


cell


windows


ensure


that


crystals


of Cr(hfac)3


did not form on


windows


render


them opaque.


Crystals of


Cr(hfac)3


were


added


to the


cell,


which


then


evacuated and


positioned


in the


cell


compartment


an IBM


UV/visible


9430


spectrophotometer.


The cell


was gradually


heated


about


C to


produce


a practical


concentration of


vapor.


was

















CHAPTER 3


TERMINOLOGY


AND CONVENTIONS


USED


IN GAS-PHASE


ION THERMOCHEMISTRY


Introduction


Values


for the energy


required


remove


an electron


from


isolated


atom,


molecule or


ion are often


obtained by using


spectroscopic


methods


that


yield


the minimum energy


required


for this


process.


This


energy


is the


adiabatic


ionization


potential


(alP)


for neutral


positively


charged


species


and the electron


affinity


for anionic


species.


used


Mass


spectrometric methods


to estimate values


for electron


and other techniques


attachment


energies


have


also been


ionization


energies


at T


K as well


for T > 0


In combination


with


other


thermochemical


data,


alP


and EA


values


provide


fundamental


information


concerning


the thermochemistry


of ionic


processes


such


as charge-


transfer


reactions


and ion solvation.


For example,


extensive


compilations


of enthalpies of


formation


of ions


at 298


K (AHf)


derived


from


spectroscopic


and mass


spectrometric


data


are


available.


Tabulated


values


for AHf


of ions


depend


on the


convention


used


to treat


the gas-phase electron.


consistently to avoid


90e,91


errors


Therefore,

in derived


a convention must


data.


be used


convention,


thermal


electron


convention


TEC),


is widely used by


thermodynamicists


treats


the electron


as a classical


ideal


gas.


stationary


electron


convention


(SEC)


"ion


convention"


is more


commonly used by


mass


spectrometrists


and treats


the electron


as a subatomic


particle


Presented


here


are definitions


some


important


terms


frequently












use.


stationary


electron


convention


is adopted


throughout


present


work,


the free energies


of electron


attachment


to the metal


complexes


obtained


in the present


work


conform to


this


convention.


Since a discussion of

energies of electron


the stationary


attachment


electron


and ionization


convention

processes


applied


to free


apparently


not appeared


in the


literature,


a discu


ssion


is give


here.


Electron Affinities


and Adiabatic


Ionization


Potentials


The electron detachment


process


for a monoatomic


or polyatomic


species Mn"

positive or


shown


(where


n is


charge


zero


negative).


M"(g)


= M(g)


The enthalpy


change


for electron


detachment


can be


expressed


as the


sum of


the enthalpy


change at


K and


the difference


heat


contents of


the products


and reactants


at temperature


T, given by


difference


in the


integrated heat


capacities


over


range


KtoT


3-2).


AHo(Mn..Mfnll)


T T
- AEo..o+ fCp(Mn+1) dT+ Cp
0 0


term AE00


is the energy


required


to form M"1


in its ground


electronic,


rotational,


and vibrational


states


from M"


in its ground


state.


When M"


a negative


ion,


AE0-0


defines


the electron


affinity


When Mn


is a neutral


or positively


charged


species,


can


- Cp(M0
0


of M"


AH(M" n M"n












If the geometries


of Mn


and M"1


differ,


will


be formed


excited


state


and the energy


required


for the


vertic


processes


be greater


than


for the


adiabatic


cess


Stationary


and Thermal


Electron


Conventions


Usually,


though


not always,


Mn and M"n


are


chemically


similar

neglect


and the difference

d with respect to


in their


that


integrated


of the electron.


heat

The


capacitie

thermal


can be


electron


convention,


however treats


the electron


as an ideal


which has


while


an integrated heat


under the stationary


electron


set to


zero.


capacity


electron


at constant


convention


pressure of


heat


relationship between


5RT/


capacity of

enthalpies o


electron


detachment between


two conventions


is given


AH(TEC)


- AH(SEC)


= 5RT/2


free energy


change


for the


process


eq 3-1 at a temperature


be written


in terms of


assoc


iated


enthalpy


entropy


changes


3-4).


AG(Mn


=- AH0Mn


- TASo(M"


-* Mn+')


value of


n in


3-4 for free


energy values derived


from


experimental


charge-transfer


The total


ASo M


entropy


is equal


to products


equilibrium


change


studies10-19,92


for the electron


- Sreactants


limited


detachment


+ S


to 0


process
(Mn+1f) -


can


- Mn+1


Mn+


SM +












Since the masses


of MI1


and Mn only


differ


the mass


an electron,


strains


for Mn1


and Mn will


be virtually


identical


these


terms


3-5 will


essentially


cancel.


eq 3-1 AStrans


is therefore


negligibly


different


from


tra 0
trans


(e-).


The translational


entropy


an ideal


particles


mass m can be


predicted


from


statistical


mechanics


by the


Sackur-Tetrode equation93


(eq 3-6)


where


V is the volume of


gas,


is the


Boltzmann


constant,


L is Avagadro


s constant


is Planck's


constant


the temperature.


5 +i (2inmkT
2 \ h2}


term A


given by


eq 3-7,


where QeLec


is the electronic


partition


function.


ASetec


= R In (Qeec(products)/QeLec(reactants))


The electron has


an electronic


degeneracy


of 2,


can


therefore


be rewritten


as eq


3-8.


ASelec


= R in (Qelec(M


/QeLec(Mn)


+ R ln2


free energy


for electron detachment given


3-2 can now be


given


according


to the two conventions.


thermal


electron


convention


includes


terms


for the


electron


, and AG


for electron


achment


is given by


Although t
experiment


Sackur-Tetrode equation


and theory


gives


for the translational


good agreement


entropy


. .


between


an atomic or .


0
Straw














AGo(TEC)


- AEo.o+ cCp(Mn1
0


T
- Cp(M)dT+ -RT
0


- T(ASrot


+ vib


+ R ln(Qeec (Ml1)/Qe ec(Mn


+ Strans(e


+ R In 2)


stationary


electron


convention


neglects


all terms


for the


electron


and AG


electron


detachment


given by eq


3-10.


AG(SEC)


- AEo.o+fCp(Mn1) dT
0


-JCp(Mn) dT
0


ASrot


+ ASvib


+ R in(Qelec(M


/Qet (Mn


3-10


temperature


at which


two conventions


give


same


value


AG(Mn -4 Mn+1


any spe


cies


can be


found by


subtracting the


right


side of


3-10


from


the right


side of


3-9 and


setting the difference


equal


zero


(eq 3-11).


5RT/2


- T(Strans


+ R In 2


3-11


Coll


ecting


the constant


terms


from the


Sackur-Tetrode equation


gives


diff


erence


between AG values


for each


convention


temperature,


-12).


[(AG(TEC)


- AG(SEC)]/J


= T(118


.3145


ln(T)5/2)


3-12


At 0


, AG


of electron detachment


is equal


to the


alP or


EA values,


there is no difference between the two


conventions


However


3-11


also


eaual


at 296.96


zero


and the two conventions


nive


identical


- T(


sw


A.












this


system the


heat


capacity terms


for H atom and


ion cancel,


ASrot


are equal


zero.


intersection


of the


lines


occurs


at 296.96


K where


the two conventions


give


same


value.


values


for each


convention


at 298 K are within


-0.02


kJ mol1


so can


be assumed


to be approximately


equal


at this


commonly


used


standard


temperature.


significance of


the result


for the


hydrogen


atom


can be


seen


in calculations


hydrogen


electrode


of the absolute


94.95


given


thermodynamic


by the


standard


potential c

free energy


standard


change


H (aq)


e (g)


= 1/2


3-13


value of


for eq


3-13


can be expressed


as the


sum of


values


for three


elementary


steps,


94,95


one of which


involves


ioni


nation


a gaseous


hydrogen atom,


eq 3-14.


H-(g


= H+(g


e (g)


3-14


value


for this


process,


and hence


value


for the


absolute


electrode


potential


(or "single


ectrode


potential")94,95,96-98


depends


ultimately on


which


electron


convention


is used.


absolute


value of


standard hydrogen electrode


used


to obtain


absolute


electron


attachment


to molecules


solution.


As noted


above,


coincidence there


is virtually no difference between


conventions


at 298 K (absolute

comparison between


= 4.44


data


This


allows


for free energies


for example


of electron


direct


attachment


to gas-
t












Also


process

to allow


shown


in Figure


appropriate

calculation


3-1 (b)


spectroscopic


a plot


data


of the relevant


of AG


is available9-101


heat


capacities,


for I2+
for 12


For this


0o
rot


The obvious


difference between


the plots of


for the


ionization


of H


atom and


is that


slope


for the


ion convention


values


opposite

and M I


of H


Under


the ion convention


is considered


results in


the SEC)


For I


a loss


a 211


and so ASOeec

of electronic


state


is formed


only the electronic


never


equal


degeneracy


from


a 'g


degeneracy


zero.


ASelec'
state,


Ionization


= R In 1/2

and ASetec


In 3


in the


SEC.


For most


small


molecules


such


as I


TAS


term


aris


from a


difference


change


between


in electronic


the alP of


degeneracy will


a polyatomic molecule


produce

and AG


largest


ionization


at T > 0


K. The difference


in the


integrated


heat


capacities


of I


differ


only


0.09


kJ mol


at 298 K and


combined


values


of TAS


only


amounts to


-0.5


kJ mol


at 298 K


somewhat


smaller than


the contribution


from


TASelec


. Further,


enthalpy


capacities


these


change


will


will


terms


arising

always

cancel


from


have


in the


the change


same


final


sign


in the


integrated


as the entropy


expression.


heat


change


The predominance of


the AS


eco0
etec


term produces the almost


linear


change


in free energy


SEC)


with


temperature


as shown


in Figure


Exactly


analogous


plots


of AG


of electron


attachment


an ion


neutral molecule under the


two electron


conventions


can be obtained by


plotting the


negative of


electron


detachment


values


given


3-10.


of electron


capture


has been


calculated


Chowdhury


combined wit


and co-workers11


spectroscopic


at 423 K from


and theoretical


the electron


data


affinity&


for the


of SO2


geometries


and


are


of Mn












The electronic degeneracy


change


is dominant


in the stationary


electron


convention,


and the dependence


of AG(SEC)


on T deviates


only


slightly


from


linearity


over


the temperature


range


shown.









13.68

13.66

13.64

13.62

13.60


13.58

13.56

13.54


a
297 K
i SEC
I


>Adiabatic
ionization
potential


9.24-

9.22 -

9.20 -

9.18 Adiabatic
ionization
9.16 potential

9.14-

9.12 -

9.10 -

9 .0 8 ,, ,, ,,,,,,,,,,,,,,,,1 11 11 11 11


* | *I | I I* S | I uI*'| I I I I *U *JI I...s|


TEC


O


C
LU
Ld
(U




















-1.04

-1.06

-1.08

-1.10

-1.12

-1.14

-1.16


TEC


-(EA


297


SEC


-1.18


0 100 200 300 400
Temperature/K


500


600

















CHAPTER


INTRAMOLECULAR ENTROPY


COUPLES


CHANGES


FO


INVOLVING COMPLEX METAL


R REDOX
IONS


Introduction


Entropy


changes


that


occur


for electron


attachment


to gas-phase


polyatomic molecules (ASa

temperature dependence of


have been


equilibrium


obtained by


constants


determining the


for gas-phase


charge-


transfer


reactions


eq 2-1)


by the


procedures


described


chapter


The entropy change


is obtained


from a


Van't


Hoff


plot


of the


data.


types


of compounds


studied


to date


have been


predominantly


organic


compounds with


delocalized


t systems,


often


containing electron


withdrawing substituents.


typically


fall


For these


in the range of


compounds,

4 cal mol1


values


of ASa0
a


are


small


An important


consequence of


equal

values


this


and constant


result


over


for the organic


is that


a wide


compounds


range of


that


and AHao values

temperatures.


have


been


studied,


are approximately


In fact,


which


are


usually measured


at temperatures


above


300 K, are typically within


kcal mol


of their


values


at 0


that


their


electron affinities


The electron


attachment


energy


data may therefore be combined with


other


compiled

without


enthalpy or

introducing


free energy data


serious


at 298 K or


at other


temperatures


errors.


It is useful


to obtain


data


for the temperature


dependence of


gas-


phase charge-transfer


reactions


involving


organometallic


coordination


compounds


since


a more


complete


understanding


of the












variety


of calorimetric


thermochemical


data


for metal-containing


compounds

difficult


energy


cycles


to obtain by more


that


provide


thermodynamic


conventional methods.


data


Examples


that


are


of the


application


of gas-phase electron


attachment


energies


energy


cycles


are given


in chapter


Organometallic


coordination


compounds


are chemically


dissimilar


to the


types of


organic


compounds


that


have been


studied


it can not


be assumed


that


values


for these


types


of compounds will


also


small


in all


cases.


Temperature dependent


gas-phase charge-transfer


equilibrium

n4-butadiene


studies


iron


involving metal


tricarbonyl


AS has


containing


been


compounds


quoted


are


rare.


to be 10 3


mol-1


considerably


higher


than


typical


values


found


for the


organic


compounds


that


have been


studied.


In principle


similar


data


could be obtained


from FTICR studies


some means


were


available


control


temperature of


the reaction


cell


and main


chamber


(Figure


2-1).


Unfortunately,


in the determination


of AG
a


for the metal


complexes


reported here,


such a


facility was


not available


and entropy


changes


could not


be measured.


Despite


the general


lack of


experimental


data


for gas-phase electron


attachment


entropies


for coordination


complexes,


data


are


available


from


other


sources.


Estimates


for certain


couples


can be obtained


from


statistical


thermodynamics


calculations


when


there are


sufficient


structural


vibrational


data.


example,


Lowenschuss


and Marcus102


have


used


statistical


mechanics


calculations


to calculate


standard gas-phase entropies


large


number of


polyatomic


ions,


including the members


of the


redox












couples


IrC63-"2/


Another


source of


data


for entropy


changes


involving


reduction


of metal


complexes


electrochemical


changes


from


values


for half-cell


redox


studies

by using

couples


of the temperature d

Scyclic voltammetry.


involving


several


ependence of

Entropy


octahedral


tris


chelate


complexes


have been


obtained


in the


laboratories


of Weaver


and co-workers,


103-105


These


studies


have


been


primarily


concerned


with


relationship between


the rate of


electron


transfer


processes


between


metal


centers


to the overall


in solution and


driving


force of


the enthalpic


reaction.


entropic

Entropy


contributions


changes


redox


half-cells


consistent


with


(ASrc0)

the sta


obtained


by the cyclic voltammetry method


tionary electron


convention


are


for dealing with


entropies

electron


of electron attachment


in the reduction originates


to gas-phase molecules,


from


the electrode


since


cannot


considered


as an "electron


Comparison o

for a particular


f data


for gas-phase


redox couple


leads


and solution-phase entropy

to the separation of the ob


changes

served


entropy


change


in solution


into an


intramolecular


contribution,


plus


contribution


from


solvent


polarization.


Such


comparisons


can not


only


provide

electron


considerable


attachment


insight


to gas-phase


into the magnitudes of


coordination


entropy


complexes,


changes


also


lead


to a greater understanding


of the role of


the solvent


in determining the


overall


change


in entropy


for a particular


redox


couple.


Presented here


are the results


statistical mechanics


calculations


of the entropy


changes


involved


for electron attachment


some gas-


phase octahedral


complexes.


The examples


given


are for complexes


that


form stable


redox


couples


in solution.
__ t- i2-


Calculations


are


repeated


- J f


and Fe(CN) 4/3"


B












performed


WCI 6

data


for the


aions


. Comparisons


obtained


here


in the couples


are made,


where


and the experimental


Ru (NH3) 63+/2+


possible,


data


, CO(NH) 3+/2+


between


reported


the theoretical


in the


literature.


insight


gained


used


to provide estimates


for the


gas-phase


entropy

M(hfac)3


changes


that


occur


for electron


attachment


to M(


acac)3


complexes.


Statistical


Gas-Phase


Redox Couples


Mechanics Applied to


Intramolecular


the Determination


Entropy Chanoes


Involving Complex Metal


Ions.


Electron attachment


to coordinated


transition metal


centers


is often


into


a metal


based molecular


orbital.


changes


in metal-ligand


bonding that

vibrations,


result

change


can shift

the moment


the frequencies

of inertia of


of metal-ligand


the molecule


skeletal

shifting


metal-ligand bond


lengths


and change ground


state


electronic


degeneracies.


These


internal


rearrangements


redistribute


internal


energy
change

solvent


of the molecule or


(gas)0).


ion and lead


total


can be expressed as


vibrational


electronic


change


an intramolecular


in entropy


sum of


entropy


in the


translational,


contributions


entropy


absence of


rotational,


(eq 4-1)


AS,(gas)0


= As


trans)0


+ ASi(rot)0


+ AS1(vib)0


+ AS (elec)0


contributions


AS1 (trans)0


, ASi (vib)0


, etc.


for ideal


gases


can be


evaluated by

Thermodynamic


using


the methods


functions


can be


of statistical


readily


thermodynamics.


calculated


from


appropriate


partition


function


Svib'


etc.)


which


a summation over


available energy


states


that


are thermally populated,


4-2.


( qt rans'












value of


q in eq


4-2 at


a given


temperature


is dependent


on the


degeneracy


of the energy


states,


and the energy


separations


between


states,


The term k


is the Boltzmann


constant.


The general


express


sion for the


rotational,


vibrational


electronic entropies


a system of


ideal


gaseous


particles


given


4-3.


(E-E0)


+ R lnq 4-3


values


of AS4(rot)0


, AS4(vib)0


and AS


elec)


for gas-phase


electron


attachment


Si(rot)


to a molecule can be


, S(vib)


and S


-(elec)


found


for both


from eq


4-3 by


the oxidized


calculating

and reduced


species and


obtaining the difference between


the values


for each


species

the two


STo evaluate the


terms on


is the thermal


energy


various


right of

and may


contributions


eq 4-3


also be


must


to ASi (gas)0


be evaluated.


separated


into


, therefore,


The term


(E-E0)


(E-E0) trans'


(E-EO)rot,
partition


etc.


Thermal


energies


function given by eq


are calculated


from the


appropriate


4-4.


(E-E0) -RT2 dln
a dT


In statistical


thermodynamics


calculations of


entropies,


expressions

treating the

oscillators.

accuracy of


used


for AS (rot)0


complex as


a rigid


and ASi(vib)0


rotor


Corrections can be made,

the calculations is small


are


approximations


and the normal modes


but the


improvements


are not included


based


as harmonic


in the

in the


discussion


here.


The contributions


from ASi(trans)0


rot)0













Chances


In Translational


Entropyv


The tr

particular

particles.


anslational


entropy


temperature and


a system of


volume


Electron attachment


related


to a molecule


ideal


particles


to the mass


of the


has a negligible


effect


the mass


the resulting


change


in AS


(trans)0


is also negligible.


evaluate


gas)0


, therefore,


no consideration need be given


AS. (trans)0


However,


values


of gas-phase entropies


of single


ions


given


this


below


purpose.


for completeness


The values


and a


reported


value of


(trans)0


in the present


required


work are


given by the


Sackur-Tetrode equation given


Stan
trans


4-5.


/3
5( 2i-mkT T
--2
2 \lh2


m is the mass


of the particle


k is the


Boltzmann


constant,


is Plank's


constant


, V i


volume of


the gas


and N


Avagadro's


constant.


Chances


in Rotational


Entropy


The expression


is obtained by


substituting


the quantum


mechanical


expression


for rotational


energy


spacings


into eq


4-2.


Since


rotational


energy


space


ings


are very


small


compared


to kT,


summation


can be


replaced by


an integral.


The result


is given


4-6.


- 82 (IBC) 3/2(2xkT)3/2
oh2


Octahedral


molecules


are classed


as spherical


tops


three


principle moments of


inertia


are the


same.


term


4-6 is


are


r(





L












octahedral molecule.


Substitution


eq 4-6 into eq


4-4 yields


following expressions


for the rotational


thermal


energy


((E-Eo)rot)


mole


degree of


freedom.


[E-E0 rot


= RT/2


For all but


the lightest


ions


and molecules


the moment


of inertia


large

value


enough

of RT/2


that


the rotational


per degree of


thermal


freedom.


energy


attains


For molecules


with


its classical


a low moment


inertia


integral


summation


and must


4-2 cannot


be evaluated


be accurately


either manually,


replaced by


or by using the Euler


Maculaurin


summation


formula.


For this


case the


thermal


energy of


is slightly


less


than


the value


4-7.


For molecules


rotational


3RT/2


applies


such as


freedom


to both


octahedral


the thermal


energy


the oxidized


complexes

is 3RT/2


and reduced


that


have


Since

complex,


degrees

value of


only


ARln


4-3 contributes


to ASi


rot)0


Substituting


eq 4-6 into eq


4-3 gives


an express


ion for AS


roto


for electron attachment


to octahedral


molecules


, eq


- 1i (Inc( (ed)-I c(ox)


( 8xkT
\ 2 )


terms


IABC (red)


and IABC


3(ox)


are the moments


of inertia


for the


reduced


and oxidized metal


complex


respectively.


In Figure


4-1 a plot


is shown


of the


rotational


entropy


, given by eq


4-3,


for the molecules MF6,


MC16


and MBr6


as a function


of increasing


distance


at 298 K.


It can be


seen


that


all the


plots


are


AStor












the plots


is close to


at 2


A and


varies


little over the


range


shown.


This


sets


an approximate upper


limit


of AS-(rot)0


- 0.6


cal mol


for octahedral metal


complexes


precise


value


can be calculated


from eq


4-8.


For the complexes


considered


in thi


chapter the M-L


bond


lengths


required


literature


data


for the evaluation


sources


are from X-ray


of rotational


and the data are given


crystallography


studies


entropies

in Table


except


were


4-1.


W-Cl


obtained


bond


bond


from


length

length


in WC16,


which


was obtained


in the ga


s-phase by


an electron


diffraction


study.


Values


are not available


for the M-L distances


in IrCl6


WC16


values


given


for the


ions


in Table 4-1


are estimated


values,


obtained by


adding 0.05


to the


values


for IrCl6


WCl5


respectively.


For both


ions the


electron


in the


lower


oxidation


state


complex


accommodated


in the


t2g non-bonding


orbital


set.


Where


structural


data


are available,


this


change


in M-L bond


length


is typical


ions


that


are stable


in oxidation


states


of similar


electronic


configuration.

these estimates


The error


is small.


introduced


in the


value


For the hexacyanoferrate


for AS (rot)0


from


complexes


rotational


entropy was


calculated


considering


each


cyano


group as


having


an atomic mass


of 26.02


amu and


situated


at the


average distance


of the C


and N


atoms


from the metal


center


(see


Table


4-1)


ammine


complexes,


rotational


entropies were


calculated


from an


effective M-L


bond

mass


distance obtained


amu.


by treating the


ammine group as


The effective M-L distance was


calculated


a single


atom of


from


appropriate M-N


and N-H bond


lengths


given


in Table


4-1.


for MX6












Table


4-1.


Metal-Lipand


Bond LenQths


in Metal


Complexes.


Complex

IrC162-

IrC1 3-
WCl6


WCl6

Co(N16 )

Co(NH3);


Bond Length/A


(M-L)


(M-L)


.307a8


.357 0


(M-L)


(M-L)


(M-N)
(N-H)
(M-L)


.114e
.010g
.173h


Co(NH3


(M-L)
(N-H)
(M-L)


8Values


taken


.936'
.010g
.995h


from ref.


Complex

Fe(CN)6


(M-C)
(C-N)


Fe(CN)6


(M-C)
(C-N)


Ru(NH3)6


(M-N)
(N-H)
(M-L)


Bond Length/A


.936a
.191a



.9008
.138a


.144f
.010g
.203h


NH3)6


(M-N)
(N-H)
(M-L)


.104f
.010*
.163h


Value
CValue

value

value


estimated by

in gas-phase

estimated by


fValue of

Value of

hEffective


distance

distance

distance


value


adding 0

from ref


adding


.05 A


value for IrCl6


(see


text)


107.


.05 A


to value


for WCl6


from

from

from


center


mass


calculated


from M-N


and N-H


distances.


.26c


.31d
















29-

28-

27

26-

'i
o 245
'*24
- 23-
0
22,

21

20,
1.8


Slope


2.98


1.9 2.0 2.1 2.2 2.3 2.4 2.5


- MBr6



-MCI6






2.MF6


2.6


M-L


Distance/A













Resulting values


Table


, with


of rotational


values


entropies

the total


for the


standard


complexes

gas-phase


are given

entropies,


ASi(gas)0


Chances


in Vibrational


Entropyov


expression


given


in eq


1
m
1-e -x


Vibrational


summation


given


energy


in eq


spacings


4-2 can not be


from a binomial


are typically


replaced by


expansion


larger than


an integral.


In eq


kT and


Equation


term x


4-9 the


hcu/kT,


in which


U is


the frequency


of the


vibrational mode


C iS


speed

can be


of light.

found by


vibrational


substituting


thermal


energy per


into eq


4-3.


degree of


result


freedom


is given


4-10.


[E-Eo


vib


x RT
ex-1
x*"


4-10


Since


the magnitude of


vibrational


energy


spacings


are typically


close


or larger than


value of


(E-EO)vib


some


fraction of


RT per


mol,


occur


per degree of

on electron


freedom.


attachment


Changes


cause


in vibrational


a change


frequencies


that


in both


and both


terms on


the right


4-3 therefore


contribute


to ASi(vib)0


Substituting


the expressions


for (E-E0)vib


into


-4 gives


expression


for As.(vib)0


4-11


w r,[


xR Xo


,_ 1


1


If


|1


E-E)Ovib


n n


R


















5.5

5.0
4.5

4.0

3.5
I

2.0
- 2.5
o

o
01.5

1.0

0.5
0.0


0 200 400 600 800 1000ob


1200


Vibrational freq./cm












oxidized and


reduced


species,


the frequencies


of which are


included


The definitions


and x0


are the


same


as that


given


but apply to the


reduced and


oxidized


species


respectively


The vibrational


characteristics


of metal


complexes


suggests


that


vibrational


entropy


changes


can be


significant


in certain


cases.


Figure


4-2 vibrational


entropy


is plotted as


a function


of vibrational


frequency


at 298 K.


It can be


seen


that


S,(vib)


increases


dramatically


as the


frequency


a vibrational mode decreases.


For


organometallic


coordination


compounds,


vibrations


associated


with metal-ligand


skeletal modes


are typically


in the


range of


100-700


-1. Shifts


these


frequencies,


oxidation


state at


of the magnitude


the metal


center,


that


can


occur


cause


for a change

significant


in formal


changes


entropy


vibrational mode,


especially


at low frequencies.


Moreover,


a non-linear molecule has


3N-6


vibrational


modes


(where N


is the


number


of atoms


in the molecule)


For MX


octahedral


complexes


there


therefore


a total


of 15 skeletal


vibrations


that


enter


into


summation


4-11.


Vibrational


frequencies used


to calculate the


vibrational


entropies


for the


complexes


considered


in thi


chapter


are given


in Tables


4-2 to


4-4.


assumption


is used


throughout


that


vibrational


frequencies


reported,


observed


in solution and


in the solid


state,


are


same


their


gas-phase values.


Since


a small


dependency


on the polari


zing


nature of


counter


ions


observed


for solid


state


spectra,


solution-


phase data


are used


wherever possible.


The only


frequencies


used


that


are obtained


from solid


state


spectra


are for the


IR active Tu modes.


These


possible


are the only


skeletal modes


IR active vibrations


for MX6


complexes


and account


for 6


12 of the


33 possible


S 0 0 0


are


* -


1












Table


4-2.


Assignments


Hexachloride Metal


of~~ ~ ~ Virrn( rrin aIi1'


Complexes


IrCl6'

IrCl6


a 353


WC16a 437


WCl6


aFrequencies


taken


from ref


111.


bThe T2 bend
16 "J6-12),


a infrared
see text.


and Raman


inactive;


value obtained


from


CFrequencies


taken


from ref.


112.


dValue


from


ref.


113.


129b


c^m~1\


of Vibrational


Fre uencies


139b


168d












Table
Metal


4-3.


Assignments


of Vibrational


Frequencies


(cm~1)


for Hexacvano


Complexes


Fe(CN


Skeletal M-C
Vibrations


M-C-N
Vibrations


C-N
Vibrations


2136


2136


2105


Fe(CN)6


Skeletal M-C
Vibrations


M-C-N
Vibrations


C-N
Vibrations


2080


2048


2033


351b


381b


350b


402b












Table 4-4.


Metal


ComIol


Assignments


of Vibrational


Frequencies


fcmn 1)


in Hexammine


exes


NH3) 6


Skeletal M-N
Vibrations


Ammonia


Rocking
Vibrations


788c


788c


Ru(NH3)6


Skeletal M-N
Vibrations


Ammonia


Rocking


769c


769c


769c


170e
270e


170e
270e


175b


788c


409d


120b
190b












Table 4-4


continued.


Co ( NH3 ) 63' a


Skeletal


Vibrations


Ammonia


Rocking
Vibrations






Co(NH3)62+,a

Skeletal M-N
Vibrations


830c


830c


187f


Ammonia


Rocking
Vibrations


654c


654c


654c


aFrequencies


taken


from ref


balue


for 06 obtained


from


6 = U5(2"1/2)


see text.


CT and T
value may
as the T


rocking vibrations
* unavailable. Fre


mode given


in ref.


are infrared


quencies


given


and Raman


inactive


are assumed


to be the


Tsme
dime


116.


Only


available


frequency


, from


, others


estimated.


228b


132b


__


k












limit


placed


on all estimated


frequencies


is 10%.


The most


potentially


serious


error


in the calculation of


vibrational


entropies


comes


from the estimated


frequency


of the inactive


skeletal


bending


mode.


The frequency


of this mode


is typically


in the


region


or so and any uncertainty produces


a large error


in the


vibrational


entropy


(see


Figure


4-2)


For most


the complexes


in Tables


4-2 to 4-


the skeletal


Tu mode was


obtained


from the


relationship


relationship


correctly predict


predicted


values


theoretically


some XY6


and has been


compounds


shown


in which


central


atom has


a closed


shell


electronic


configuration.


octahedral


transition metal


hexafluorides,


for which


is available


from combination bands


relationship


is generally


or resonance


observed


phosphorescence


to hold


to within


spectra, t

the error


limits


10% given


here.


Ru(NH3)6


data


not been reported,


for the


except


frequencies of


for the Tu mode


(see


skeletal modes


Table


have


4-4)


frequencies


given


Table


4-4 are estimations


based


on calculations


observed


frequencies


for other


hexaammine complexes of


ions.


addition


to the skeletal modes


given


for the


hexaammine


complexes


Table


4-4,


frequencies


are reported


for the


ammine


ligand N-H


rocking


vibrations.


Unlike


skeletal


modes,


these


frequencies


are


available


for Ru(NH3)62.+ 116


The N-H rocking vibrations


are


the only


other


vibrations


of low enough


frequency to contribute


significantly to


vibrational


entropy


for these complexes.


There


are a total


of 12 modes


and T2 )


of which only the


and T2g modes


are


infrared


Raman


inactive.


These


frequencies


are observed at


830cm


respectively


for Co(NH3)63


but only the


infrared


active


Tu modes


1/2)


are


= u5(


(Tlg,












Chances


in Electronic


EntroDv


Electronic energy


separations


are usually


large


compared


to kT


exponential


term


4-2 is therefore close


zero


usually


equal


to the degeneracy


the ground


electronic


state


maximum possible degeneracy


an electronic


state


is given by


product


of the


total


spin


and orbital


degeneracies.


required


information


carried


in the


spectroscopic term


symbol


for the


state.


Under the octahedral


symbol


point


group the orbital


singly degenerate


, doubly


degeneracy

degenerate;


denoted by


, triply


degenerate.


total


spin


degeneracy


(multiplicity


given by


2S+1


where


S is the


total


spin


angular momentum and


is denoted


in the


superscript


preceding the orbital


symbol.


spectroscopic


term


symbols


the metal


for octahedral


d orbitals


complexes


in an


are derived


octahedral


ligand


purely

field.


from


symmetry of


In reality


, the


degeneracy of


the electronic


ground


state


a particular


complex


can be


split

of the


n energy

complex


The extent


the nature of


the splitting


the electronic


depends


state


on the


structure


itself.


ground


state


electronic


degeneracy


of the complex may therefore


be less


than


value


suggested by the


spectroscopic


term symbol,


since


it is


dependent


on the thermal


population


the energetically


split


states.


There


are two


principle effects


that


contribute


to the


splitting


orbital


degeneracies,


spin-orbit


coupling


and distortions


from perfect


octahedral


symmetry.


Spin-orbit


couplin.


coupling of


spin


and orbital


angular


moment


of the d


electrons


results


a splitting of


electronic


degeneracy


For A and E ground


states


there


no orbital


angular


momentum


consequently


no spin


-orbit


coupling


For T ground


states


qetec












state,


value of


the Racah


electronic


repulsion


parameter


the magnitude of


ligand


field


(10Dq)


For second


third


metals


the effect


is greater


and orbital


splitting


are generally


in the


range


of 500 5000


Distortions


from octahedral


symmetry.


Molecules


that


have orbitally


degenerate ground


states


have


a tendency to


physically


distort


to move


to a


state of


lower


energy


and lower


symmetry,


removing the


orbital


degeneracy

Jahn-Teller


of the state.


theorem.


119,120


This


statement


The effect


a simplified


is observed


form of


to occur


octahedral metal


ground state


complexes

not purely


For example,


octahedral


Ti(H20) 6


and the


which


has a 2Tg


orbitals


are


split


into a


set of bl,


states.


single electron


resides


in the lowest


energy


orbital.


Similarly,


for ions


such


, a pure octahedral


ligand


field


would


produce


a degenerate


of orbitals


containing three electrons.


From experiment,


the orbitals


found


to be split


such


that


the unpaired


electron


resides


a state


single


orbital


degeneracy.


The magnitude of


the orbital


splitting


typically


in the


range of


about


several


hundred


wave


numbers


up to


about


2000


(observed


for Cu2+


complexes)


so are comparable


to the


magnitudes

complexes


of splitting


symmetry)


due to spin-orbit


a trigonal


distortion


coupling.


is possible


tris-chelate


if the


"bite


angle"


of the ligand


not precisely


In this


case,


the degenerate


set (octahedral


symmetry


is split


into a


set of


orbitals.


set in octahedral


symmetry.


M(acac)3


An example of


complexes


has been


symmetry remains


the degree of


reported


a doubly

trigonal

Co(acac)3


degenerate

splitting


E set in


crystal


structure of


Co(acac)3


reveals


an average O-Co-O bite


angle


of 97.3


A'


. S 1 -.


row


are


+ eg


q


4R












The geometry


of the complex


in the gas-phase


has not been


reported.


The entropy


an electronic


state can


be evaluated


from


4-3.


the absence of


a thermally


acce


ssible


higher


lying


states,


entropy


of a ground

degeneracy


states


of the


electronic s

of the ground


is possible,


thermal


tate


is given by


state.


the entropy


population


If thermal


of the


of the split


= R In


population


state must


states;


where g


of higher


be considered


that


the

lying

in terms


electronic


partition


function


(qetec)


must


be evaluated.


An orbitally


split


state


that


can be


thermally populated


will


also


posess


a thermal


energy


((E-Eo) etec)


This


thermal


energy must


also be


considered when


evaluating the entropy


of the state


, as defined by equation


4-3.


example of


the relationship between


the entropy


an electronic


state


and the


splitting of


temperatures


AE = 0


is given


entropy


the degeneracy


in Figure

given by


of the


state


4-3 for the example of


= R In


and for a 2E


an energy


a 2E


state


at two


state


this


equal

shown


to R In 4


75 cal


in Figure


mol"


As AE


splitting of


increases


value


state


gelec'


given by


approaches


+ e-ERT),

a value of


rapidly decreases


R In


(1.38


and R


cal mol


rapidly


as shown


in Figure 4-3


AE > 0


approaches

thermally


(E-Eo) etec


as AE becomes


accessible.


increases


so large

value of


, reaches


that


(E-EO)elec


a maximum


upper


value,


state


and then


no longer


is given by


+ e(AERT))]


AE and has been


shown by


Lias


and Ausloos92c


reach


a maximum of


-0.2


kcal


mol-1


for a 2E


state.


contribution


the entropy


is given


from


(E-E0)etec/T,


which


is also


shown


in Figure


overall


effect


on the entropy


of the


state


is that


it also


converges


on the value of


R In


, but retains


a significant


amount


= 2(


qelec


qe te


[(e(-AE/RT)) /










































350
298


U-


R Inq


(R





(E-Eo)/'T


- -
a- -t


- a-
C--


0 200


a


e-...


=












splitting.

states is


At absolute


possible


zero


no thermal


and the entropy


of the


population


state


of higher


given


lying

the


degeneracy of


lowest


lying


state.


Assess


inar the entropies


of electronic .round


states.


combined


effects


give


of spin-orbit


a characteristic


coupling

splitting


and distortions


of electronic


from octahedral


symmetry


degeneracy


particular metal


complex that


dependent


on the metal


ion,


oxidation


state


and the nature of


coordinated


ligand.


Since


energy


spacings


are typically


on the order


of magnitude


of kT,


a range


of states


in the electronic manifold


can be thermally populated,


depending


on the


resulting


energy


spacings


and the temperature.


increasing population

particularly manifest


susceptibility


of higher


lying


states


with


temperature


in the temperature dependence of


transition metal


the magnetic


complexes.


In Table


4-5,


estimates


are given


for the


change


in electronic


entropy


for the redox


couples


containing the


octahedral


ions


cons


idered


here.


For the


complexes with A and E


ground


states,


the orbital


angular


momentum


spin-orbit

electronic


function


orbital


is quenched


coupling.

state was


equal


and so the electronic


For A ground

estimated by


to the spin


degeneracy may


be split


states,


degeneracy

therefore,


assuming that


degeneracy.


by distortions


is not split

the entropy


the electronic


For E ground


partition


states,


from octahedral


symmetry.


The entropy of


state depends on


the thermal


population


of the


upper


state.


separation


As shown


in Figure


between


4-3,


states


for typical


, the


upper


values


state


of the


can be


energy


accessible


ordinary


temperatures.


Since the


energy


spacings


between


split


states


are not known,


the entropy of


E states was


estimated


from












Table 4-5


Electronic


Entropy Chanqes


For Redox Couples.


Change in
Electronic


Redox


Couple


Ground


State


soln)


(elec)'

.78 1


IrCl6

WCl60


(soln)


(soln


-1.78 1


1 -0.58 2


Co (NH3 ) 63+/2+

Ru (NH3) 63+/2+


(soln)

(soln)


1 2

1 -1


.48 2


.78 1


Sc(acac)


acac


V(acac)


Cr(acac)
Mn(acac)

Fe(acac)


acac)


(gas)
"(gas)

(gas)


.78 1

.41 3

.58 2

.15 0

.34 0


0/-(gas)


(gas


(gas)


.87 + 2

.48 2


(gas)


8Values


are


convention.


in cal mol"1


Entropy of


Calculated


the free


electron


ox + e


- red


is not included


in the


'e(CN)63-/4-


-, 3T

- 4T


2T2
-* 3TI

4 A2
- 5E

-, 6A1


- 4T1












states


split


spin-orbit


coupling


can have


a lower


degeneracy than


spin


degeneracy122


and a realistic evaluation


of the


range of


values


the electronic partition


function


is difficult


to estimate.


For the


complexes


that


have


ground


states


in Table


4-5,


the electronic


entropy


estimated


from a


value


the partition


function


taken


as the


average of

appropriate


to the maximum degeneracy of


term symbol.


Estimates of


state,


electronic


as given by the


energy


spacings,


hence


the electronic


partition


function,


can be obtained


from matching


the observed


complex


temperature dependence of


to the theoretical


temperature


the magnetic

dependence,


susceptibility


derived


from


theoretical


energy


spacings,


but such


data


are scarce.


Comparison


of Solution-Phase


Redox


Couples


and Gas-Phase Entropy


Involving Octahedral


Metal


Changes
Complexes


for Some


Entropy Chanaes


for Solution-Phase Redox Couples


The experimental method


for obtaining


entropy


changes


for half-cell


redox


couples


involves


the use


of cyclic


voltammetry


in a non-thermal


cell


arrangement


that


permits


temperature of


half-cell


containing


the redox couple of


interest


to be varied,


while the


temperature of


the other


half-cell,


containing the


reference


electrode


is held


constant.


The method


provides


a simple means


to evaluate


difference between


the absolute


ionic


entropies


of the


reduced and


oxidized


halves


of the couple


given


AS,


0
- Srad -


4-12


Interpretation


of entropy


changes


for redox


couples


involving


was












complex


ion.


However,


dielectric


continuum models


have not


provided


adequate description


of observed


o values.


In particular,


anomalous


entropy


changes


assoc


iated with


Co(III)/Co(II)


couples


comparison t

quantitative


for redox

mechanics,

estimated.


o analogous

explanation


couples


Ru(III)/Ru(II)


involving


intramolecula


y calculating

coordination

r and solvent


couples)


have evaded


intramolecular


complexes


satisfactory


entropy


by using


contributions


changes


state istical


to ASrc


can be


Several


values


have


been


determined by


various


workers


selection of


results


for various


redox


couples


are given


in Table


4-6.


solvent


for all redox


couples


in Table


4-6 is


water.


Also


given


in Table


4-6 are the


theoretical


values of


0 predicted by the


Born


equation


(ASBorn0).


Born equation124


is based


on a purely


electrostatic model


can be used


to obtain


the change


in free energy


entropy when a


charge


is transferred


from a


conducting


sphere


vacuum to


an identical


sphere


in a medium of


dielectric


constant


e (eqs


4-13


4-14).


AGnorn


- -_ 21--
2r\ e


4-13


AS Brn


- -P
^ BoX 1
" t aT ) p


- q2 1ne
2iTe 81nT J p


4-14


In eqs


4-13


4-14,


the charge on


the conducting


sphere


e iS


the dielectric


constant


of the medium.


When


the medium


water


at 25C


written


spheres

in the


are


ions


convenient


of absolute


forms of


charge


4-15


ze, eqs


4-13


4-14


can be


4-16.












Table


4-6.


Experimental


and Theoretical


Entropies


Redox Couples.


Redox Couple


ASBorn


0
ASBorn0)


A (M-La .)
D iasance


Ru (NH3) 63+/2+' C

Os(NH3)63+/2+


Co (NH3) 63+/2+


Ru(en)33

Co(en)33

Ru(H20)6


18.5


18 0


d 45


+/2+


13 0


37 2


3+/2+,c


38 3


14.6


14.6


14.6


13.0


13.0

14.6


14.6


CO(H20)3+/2+,d
Fe(H20)33+12+,b


bipy) 33+12+


14.6


28.4


c 1


-0.048f


Fe(bipy)33+12+,c

Co(bipy)33+/2+, c

Fe ( CN) 63" 14 -, c


22


-41.5


aAll


values


given


in cal mol


bata

CData


from ref

from ref


dFrom ref


eData

Data


103.

104.


. 105 (value estimated by


from Tabi


from ref.


authors)


4-1.


123.


.040e


0.178e


+/2+,b


-0.036e













ASforn


z2
- -9.649 -z-
r7A


cal mol-1


4-16


Born


equation


is most


successfully


applied


to large


approximately

solute/solvent


spherical


ions


interactions


of low charge,


are absent.


and where


For these


specific


ions,


the effect


changes


in size of


the ion with


changes


in the oxidation


state of


metal


the effect


of dielectric


saturation


are both minimized.


should be


noted


that


for the


reduction of


a complex


bearing


a positive


charge

bearing


sign


of ASBorn


a negative charge,


positive.


sign


For neutral


of ASBorn


complexes


is negative.


those


A more


positive entropy


can be associated


with


ions


of lower


charge,


since


there will


be less


"ordering"


of the surrounding


solvent


molecules.


Comparing the experimental


and theoretical


entropy


changes


for the


redox


couples


in Table


4-6,


it is


seen


that


there


is generally


a poor


agreement


between the


two values.


However,


the theoretical


value


serves


as a reference


point


to which


the experimental


values


can be


compared


absence of


specific


solute-solvent


interactions.


sign


magnitude of

information


the deviations


about


the nature and


of experimental


the extent


results


of the


from ASBorn


changes


o provide


in specific


solute-solvent


interactions


that


occur


on reduction


a particular


metal


complex.


The difference between


ASBorn


for each


couple


is included


in Table 4-6


for this


purpose.


It is particularly


interesting to


note


in Table


4-6 that


for the


couples


Ru (bipy) 3+/2+


(where bipy


'-bipyridine)


which


bonding,


the nature of


that


the M-L bonding


value of


is more


- ASBorn


complex


than


is negative.


simple


For each


of these


Fe(CN) 3-4-












energy match


with


ligand


r orbitals,


and subsequently


an overall


increase


in the degree of


and Weaver


to account


(M-L)


for the


bonding.


negative


A related


value


argument


of ASrc


was


used by


- ASBorn


the Ru(bipy)332

are in operation.


couple.


was suggested


The water molecules


close


that


to the


competing


effects


ruthenium center,


including

therefore


those

less


surrounding the


"ordered"


ligands,


in the lower


will


oxidation


be less

n state,


polarize

giving


rise to


positive


contribution


to ASr


However


, the water molecules


adjacent


to the bipyridine rings may experience


an increase


in polar


ization


going to


the Ru(II)


state


since


the added


electron


will


significantly delocalized


around


aromatic


rings,


acting to


increase


their


net charge den


sity.


The latter


contribution


would


give


a negative


contribution


to ASrc


An opposite


effect


was


used


'to describe


anomalously


large value of


for Co (bipy) 33+/2+


Co(III)


Co(II)


reactions


involve


the electronic


convers


ion t2g


- t29g


which


should minimize


the extent


of electron


delocaliz


action


in the


reduced


state


and therefore discourage


crease


solvent


polarization


the vicinity of


the bipyridine


rings.


Further,


the expansion


at the


metal


center was


suggested


to lead


an especially


large decrease


polarization


for the


negative


of nearby water molecules.


value of


- ASsBorn


It seems that


o for the Ru(bipy)33+/2+


e arguments

couple are


not without merit,


conceive.


since


The explanation


an alternative explanation


for the Co(bipy)33+/2


is difficult


couple may


questionable,


cal mol"1


however,


between


since


the large difference of


the Ru (bipy)33'+/2


and Co (bipy) 33+12+


approximately


couples


consistently


found


for other


couples


involving


reductions


at Ru(III)


CO(III)


centers where only


M-L


o bonding


is possible.


seems


that












Intramolecular


Contributions


to ASrc
rc-


Single


ion hydration


entropies


have


been


obtained


for many monatomic


polyatomic


ions


by evaluating the


entropy


change


for the


transference


a gas-phase


ion M of


charge


n to the


solution


phase


according to

the reaction


the reaction Mn(gas


- Mn(aq


) 125-128


value of


given by


AS (hyd


i (gas)


i(aq


4-17


o+ 6.35 cal mol-1 K-1


value of


6.35


cal mol-1 K-1


(R In 24.41)


arises


from the different


standard

Si(gas)0


states


and Si(aq)


for the gas-phase


for a particular


and the


solution


phase.


ion are typically


value of


quite


different,


value of


S1(aq) being


smaller


and often


negative.


Translational


freedom


is restricted


and it is uncertain how


rotational motion will


affected.


Also


the difference


polarization

in entropy of


of the solvent may


an ion between


contribute greatly to


phases.


Although


aq)


data


are available


for a large number


polyatomic


ions,


there


are apparently no


aq)


for ionic


reports

species


on comparisons made between ASi(gas)0


in redox


couples.


results


of the


calculations that


yield


gas-phase


entropies of


the octahedral


complexes


considered


in the


present


work are


presented


in Table


From


the results


of the calculations


it can be


seen


that


trans)0


same


within


for the oxidized


cal mol


and reduced


(Figure


4-1)


species,


that


Si (rot)0


can be anticipated


that


these


terms


will


also


remain


constant


between


the oxidized


and reduced


species


in solution.


Much


larger


differences


in entropy


can potentially


arise from S


vib)


and Si


elec)0


, and the


calculated


gas-phase


values














AS


- ASi(vib)


o + ASi(elec


0+ ASoolv


4-18


4-18,


ions


in the


ASsolv

redox


is the difference


couple.


There


in the entropy


are two situations


solvation


of the


where the


contribution


o to


can be


separated


from ASrc


so that


contributions


of large


radii


from AS.

, ASsoLv


vib)


and AS.(elec)0 may be estimated.


in water


predicted by the


Born


equation


ions


(4-16)


to be small


comparing


and therefore


two redox


couples of


= AS1(vib

different


+ AS4(elec)


metal


ions


Also,


when


coordinated by the


same


ligand,


and undergoing the


same


change


in oxidation


states,


Assolv


is constant


and AASrc


= AAS.(vib)0


+ AAS.(elec)0


It is illuminating to


calculations,


obtained


which


aqueous


compare the


are given

solution,


in Table


which


results of


4-7,


are given


gas-phase


to the experimental


in Table


4-6.


results


For the


ions that


form


the redox


couples


Ru (NH3) 63+12+


, IrC12"


, WCl6


the difference


in gas-phase entropies


is small


therefore


only


Ssolv0 will


ASi(gas)0

value of


= 17.8


gas)0


contribute

cal mol"1 E


can be


to ASrc.


For the Co(NH ) 3+/2+


The origin


traced


of the


to the difference


couple


comparatively


in spin


large


states


between


bonding

result


the oxidized


and reduced


significantly weakened


of the doubly


occupied


species.

relative


In Co(II)

to Co(III)


antibonding e metal


comply


exes


complexes


based


orbitals


the M-L


as a


in the


Co(II)


state.


As a result,


the skeletal


vibrational


modes


are shifted


to substantially


lower


frequencies


and a large


increase


in vibrational


entropy re

also gives


sults.


rise


The greater

a significa


electronic

nt increase


degeneracy


ASi(elec)0


of the Co(II)


(Table


state


4-6).


1wL I -. *


t I e~.


S U


Fe(CN) 3"/4"























































C o
M -
1 0

w in
-4 9.-
o
rS Ca


to o
* .
0,4
OH-M

40 (^ r-4


,-* I
e1
0I I
CE


r-i |
I I
rEO u


+ +1"


(Ned


n9 (n -











(1,10-phen)

frequencies

observed at


complexes


of the


For Fe(bipy


384 and 367 cm-1


same metal


infrared


ion have


active M-N


For Fe(bipy)32+


similar M-L


vibrations


these


vibrational


are


frequencies


are


shifted


slightly to


386 and 376


respectively.


Co(1,10-phen)3


similar


frequencies


to these


are observed at


378 and


cm-1,130 but

frequencies


for Co(bipy)3


are shifted to


at 266 and 228 cm"1


Large


substantially

contributions


lower

to vibrational


entropy


can be generally


expected


for cobalt


couples


that


undergo


same change

vibrational


reduced


in spin


stat


frequencies


species


are


e, although a


that


required


complete


are different


set of data


between


to quantitatively


for all the


the oxidized


evaluate As


(vib)


all the


same


redox


spin


couples

change


involving Co(III)/Co(II)


involved


and values


reductions


for As rc


are


in Table


constantly


25 cal mol


higher than


corresponding Ru(III)/Ru(II)


couples.


The difference

and Ru(NH3)+2


15.4


in the values of


obtained


cal mol1


intramolecular


ASi (gas)0


for the


from the calculations


result


entropy


changes


demonstrates


for Co(III)/Co(II)


couples


Co(NH3)63/2


in the present

importance of


redox


work


couples,


offers


a feasible explanation


and Co(III)/(II


of the large differences


redox


couples


studied


in ASrc

aqueous


for the


solution.


Relationship


Between


Free Enercv


and Enthalov of


Gas-Phase Electron Attachment


to M(acac)3


and M(hfac)3


Complexes


The electron


attachment


energies


quoted


for the M(acac)3


M(hfac)3
at 350 K.


complexes


The data


in the present


would


serve a


work are


wider


free energies


range of


(AG )


obtained


applications


I- harmnr~hom nr Ae0 RnA AR0 nan I A ha nhl- ni nail n* al-bar


if vanluia nf


Ru(III)/(II)


A o n/I AW 0












data.


Values


for AH


at 298.15


K can be


readily


combined


energy


cycles


with


compiled


data


for other


processes.


The relationship between


an experimental


temperature


(Texp)


and AHl
a


and AG0


different


temperature


is given


in equations


4-19


4-20


AG (T


- AG(T


+ AS0 [Texp


T
- Cp(M)dT+
T


T1
fCp(M-) dT
T


4-19


AH (T)


- AG(Te,


+TexpAS


T
f Cp (M) dT
T


T
fCp(M-) dT
T


4-20


It is often assumed


that


temperature dependence


and AS


electron


attachment


or ionization


a neutral molecule


is negligible


For example,


for electron


= the electron


used by


capture

affinity


Kebarle to quote electron


a species


affinities


M to


form M ;


equation

E organic


4-20


AGe(0


been


compounds,


neglecting


the integral


terms.


Lias


and Ausloos92c


have explored


validity


of this


assumption by performing


statistical


thermodynamics


calculations


on several


organic


and inorganic


compounds


from


spectroscopic data.


As shown


above,


and stated more explic


itly


Lias


and Ausloos, t

energy between


he difference

a species M a


in translational

nd its ion (M o:


and rotational


r M')


thermal


is negligible.


Differences


can only


arise


from


(E-Eo)vib


(E-EO)etec


Under the


convention


volume of


the electron


zero


so for electron


attachment


AP =AV


= 0.


Therefore,


- Cp


and AE


= AH.


Lias


Ausloos


showed


difference


that


between


from the compounds


adiabatic


they


ionization


studied,

potential


largest


enthalpy


of ionization of


enthalpy


at 350


arose


for ethylene,


which


a a- o 1 -












4-20


can be


expected


to be significantly


greater than


absolute


magnitude of


sum of


the integral


terms.


From


calculations given


above


for pairs of


octahedral


transition metal


complex


ions


that


form


redox


couples


(Table


4-7)


can be


seen


that


where


acceptor


orbital


is a non-bonding metal


is 3


cal mol -1


This


value


is comparable


determined


capture

From the


to the organic


experimental

the M(acac)3


results


of the


compounds


for which As a


can be expected


and M(hfac)3

calculations


to apply


complexes of


given


in Table


Sc, Ti,


been


for electron


V and


4-7, for the gas-


phase


Co (NH3 ) 63+/2+


couple a


larger value


of ASao


cal mol"1


obtained


, which


was


attributed


a consequence


of the difference


in d


electron


difference


Co(acac)3

range of

electron


configuration between


in electronic


the oxidized


configuration


Co(hfac)3


20 cal mol"1


capture


results


couples


For the


acac


in the following


and reduced


is expected


and ASa may


and hfac


changes


forms.


to exist


also


complexes


same


for the


be in the


of Cr and Mn


in d electron


configuration;


Cr t2g


-t29


Mn, t2g


In each


case the


additional


electron


is accommodated


in the antibonding


set and


be in the


range of


- 20 cal mol"1


Conclusions


results


of the


calculations


presented


here demonstrate


importance of


intramolecular


entropy


changes that


occur


on electron


attachment


to coordination


complexes.


For solution-phase


redox


couples,


intramolecular


entropy


changes


are generally


smaller


than


entropy


change occurring


in the surrounding


solvent


However,


in special


cases


was


-t2g












such


as the Co(III)/(II)


changes


redox


contribute


couple,


even


couples


considered


significantly to


lons


of quite


here,


the total


small


radii.


intramolecular


entropy

In the


change


case


entropy

for the


of redox


couples


involving


large


ions


such


as Co(bipy)33+/2+


differential


solvation


effects


are expected


to be


relatively


small


probably

change.

cal mol-


be attributed


almost


For example,


and 22 cal mol


entirely to


for [
-1 K-1


Fe(bipy)3]

in water,


an intramolecular


3+/2+


entropy


[Co(bipy)3]3+/2+


respectively.


Although


extensive calculations


of the vibrational


partition


functions


cannot


carried


-20 cal mol


view of


out for these


-1 K-1


ions


difference


changes


in M-N


to the


in the ASr


stretching


lack of


o values


(and


spectroscopic data,

is understandable


presumably bending


frequencies


that


occur


for these two


couples.


Essentially no


change


vibrational


frequencies occurs


for (Fe(bipy)3 ]3++


while


frequency


change


for [Co(bipy)3]3]+2+


couple


amounts


an average


-130


insight


changes


that


gained


has enabled rough


occur


from the


calculations


estimates


for gas-phase electron


to be made


attachment


for intramolecular


for the entropy


to the


entropy

changes


transition metal


1-diketonate complexes


investigated


in this


study.


can


are

















CHAPTER


METAL-LIGAND
FOR GAS-PHASE


BOND


ENERGIES


TRANSITION METAL


AND SOLVATION


ENERGIES


TRIS(ACETYLACETONATE


COMPLEXES


AND THEIR ANIONS


Introduction


There


have been


several


attempts


to determine


average


homolytic


and heterolytic


M-0 bond energies


in M(acac)3


complexes.


most


reliable


are obtained


through a


thermochemical


cycle based


on the


enthalpy of

calorimetry.


hydrolysis


the complexes,


In the auxiliary thermochemical


obtained by using


data required


reaction


in the


cycle,


value


for the


homolytic


bond


enthalpy


of the enolic O-H bond


in acetylacetone


introduces


the greatest


uncertainty,


since no


experimental


data


is available.


From


the results of


thermal


gas-phase


charge-transfer


reactions


involving


acac


Sions


presented


here,


proton

of this


affinity


acac


bond energy.


previously


reported,


From the original


reaction


a new estimate


calorimetry


can be made


data,


better


estimates


heterolytic


bond


can


then be made


energies


for M(ac


for the average

ac)3 complexes.


e M-O


This


homolyti

s data,


combined


with


the gas-phase electron attachment


data


for the M(acac)3


complexes


and the free gas-phase


ion,


leads


to the


average


heterolytic


bond


energies


in the corresponding M(acac)3


Sions.


Several


the M(acac)3


complexes


for which


electron


attachment


data


were obtained


also exhibit


reversible


electrochemical


behavior


one electron


reduction.


From El2


data,


estimates


can be made of


single


a 1 a 4. aA a a -. A 1 a fl ante a --- I .L. t.


L -- A L --


^ _- _< _












essential for

processes at

contributions


a complete


understanding


coordinated metal

of the changes i


of the


centers.


n solvation


thermodynamics


Consideration


energies


of th


and bond


of redox

e relative

energies


that


occur


for electron


attachment


to M(acac)3


complexes


provides


overall


related


appreciation of


to the magnitude of


how ionization


a particular


potentials


M(acac)3


M+3(g)


redox


ions


are


couple.


Electron Attachment


Enerov Relationships


The general


thermochemical


cycle


in Figure


5-1 is the basis


most


of the


thermochemical


results


presented


in this


report.


The cycle


shows


the general


functions


thermochemical


for M-L bond


formation


relationships between


or solvat ion


thermodynamic


a complex


electron attachment


three


thermodynamics


physically different


(AXa)


environments


for a metal


(reactions


ion in essentially


b and


given


temperature.


A cycle of


this


type


and crude estimates of


various


thermodynamic quantities


were discussed by


Buckingham and


Sargeson


some


years


ago.


In reaction


electron


attachment


to a metal


ion M with


charge


z in the free gaseous


state,


MZ(g


electron


attachment


is to the gas-phase


complex


of charge


n in which


the metal


ion M


is equivalently


ligated by


anionic


ligands


In (c)


solvated metal


complex


is reduced


to (MLy]n-1(soln)


For the M(acac)3


complexes


considered


here,


L=acac,


= 3,


= 0.


In the


upper


part


of Figure


, labelled


the difference


for the electron


attachment


reactions


(a) and (b),


given by


AXao[MZ(g


and AX[ MLyn(g)


are thermochemically related


to the


= +3,

















































































W.>.


J












In the


part


of the cycle


labelled


II the difference


in AXa


for the


electron


attachment


reactions


AXa [(MLyn(g


and AXao[MLyLn(soln)]


thermochemically related


to the difference


between AX


solvation


the oxidized


and reduced


forms of


the complex


AXso v[MLY


AXsotv [MLy


(n-1)])


Experimental


ionization


results


of a neutral


for reaction


or electron


usually


attachment


involve


a neutral.


incorporate


the energies


for these


processes


into


thermochemical


cycles,


the values must be determined


under thermal


conditions.


Such


data


be obtained by using mass


spectrometry through


studies


of electron-


transfer


equilibrium reactions


and are often


used


to estimate


adiabatic


ionization


energies or


the electron


affinities


of polyatomic


species


(these quantities


of formation


The method


of the neutral a

d can be applied


strictly the

nd its ion at

equally well


differences between


as discussed


to reactions


the heats


in chapter


involving


positive or negative


ions,


but such


data


for metal


complexes


scarce.


Vertical


ionization


data


are more widely


available


for volatile metal


complexes


photoelectron


adiabatic


from studies of


spectroscopy

and vertical


ionization


(PES).


appearance


56,131


processes


potentials


The energy difference between


can be


relatively


small


if the


geometry

adiabatic


of the neutral


ionization


is similar to


energy


that


for ferrocene


ion.


has recently


been


example,

estimated


.69 eV by


FTICR133


(Mautner134


suggests


6.81


eV from


pulsed


high


pressure mass


spectrometry


studies),


while


vertical


IP value


obtained


from PES


6.88


Significant


differences between adiabatic


verti


cal energies


arise,


however,


when geometry


changes


upon


electron


attachment


can












7.0 eV


For metal


complexes,


therefore,


vertical


ionization


data


only


be used


in thermochemical


cycles


such


as Figure


5-1 for those


cases


where


it is known,


or can be


reasonably


assumed,


that


the geometries


the neutral


and the ion


are not too dissimilar.


It should be


noted


that


even


if the


0-0 transition


energy


(the


adiabatic energy)


can be obtained


from the


PES spectrum,


a statistical


mechanical


calculation must


be used


to derive enthalpy,

temperature. Spect


entropy,


and free


roscopic data


needed


energy


changes


for such


a given


calculations


are often


unavailable or


incomplete


for transition metal


complexes.


On the other


hand,


electron-transfer equilibrium


studies


provide data


that


can be


used


directly


in thermochemical


calculations


involving


ioni


zations


electron


attachments


near


room temperature.


Combining cas-phase electron


attachment


energi.es


with


other


thermochemical


data.


In order to


combine thermal


gas-phase


electron


attachment


energy data


for M(acac)3


complexes with


other thermochemical


data


it is useful


to know the


temperature dependence


of Ke


2-2)


since


such


data


leads


to values


of ASa


and AHa


Estimates


of AG
a


other


temperatures


can then be made.


From


the conclusions


drawn


from


chapter


concerning the magnitudes


of ASa0


for gas-phase


coordination


compounds,


a maximum


value of


= 20 cal mol'


for the


reaction


Co(acac)3(g)


- Co(acac)3


is assumed.


value


for AHa
a


at 298


K is


-7 kcal


mol"1


higher


than AG8


at 350 K (assuming


independent


of temperature).


For the other


M(acac)3


couples


where


less


change


in vibrational


and electronic


entropies occur,


ASaO


should


be smaller


Values


similar


for the


to the values


total


reported


for organic


metal-ligand heterolytic


bond


compounds.


dissociation


enthalpies


for M(acac)3


complexes


(AHhet


(M-(acac)3]


of the


first


S


can












using the


approximation AG


= AHa


(298


K) is small,


since


these


bond


enthalpies


have


values


in the range of


-600-650


kcal


(see


below)


When quoting


an average energy per


bond,


AXhet


(M-O)


error


introduced by


assuming


- AHa


is probably


kcal


cases


As discussed above,


throughout


dissertation


stationary


ectron


entropy

of AHa


convention


zero


is adopted,


to the


for monoatomic


which


free electron.


ions


at OK


apply


assigns


a heat


Under this


capacity


convention


at all temperatures,


values


since


heat


capacities


of Mz


and M(z'1)


are always


equal.


Therefore,


values


AHa M+3(g)

potential


are given by the

for the metal. T


negative


value of


stationary electron


third


ionization


convention


is adopted


to maintain


organic


consistency with


reference


the original


compounds on which


AGa values


the results


quoted


presented


for the

Ln this


dissertation


are based.


From cycle


II of Figure


5-1, AG0 M(acac)3(g)


data


can be


compared


with AGa

between t


for the


same process


solvation


free


in solution


energies


to yield


of a M(acac)3


the difference


neutral


its anion.


Values


of AGa M(acac)3(soln)


can be estimated


from electrochemical


values


for M(acac)3


AG8[M(acac)3(g)]


couples


data


(see discussion below).


at 350 K is valid at


Assuming


other temperatures


again


introduces


an approximation,


but using the


upper


limit


of -20 cal


for ASa0
a


, the error


introduced


in quoting values


AAG sov


acac)3


In the


at 298 K is again


thermochemical


used


typically


kcal


in this work


mol"1


to obtain


values


the bond


dissociation


enthalpies


for M(acac)3


and M(acac)3


complexes


value


for AHa


at 298 K for


acac"


radical


is needed.


value obtained












Homolvtic


and Heterolvtic


Bond


Enthalpies


in M(acac)ygql


Complexes


and M(acac)3


(c) Ions


The difference


total


heterolytic


or homolytic metal


-ligand bond


dissociation


enthalpies


between


any M(acac)


complex


and its


negative


ion can be obtained


from the relationships


AAHhet [M-(acac)


- AHhet


[M-(acac)3]


- AHhet[M-(acac)3


= AHa[M+3(g)]


- AHa[M(acac)3(g)]


AAHhom,[M-(acac)3


= AHho[M-(acac)3]


= AHa[M(g


- AHhmo[M-(acac)3 ]


- AHa[M(acac)3(g) ]


5-1 (b)


Values


for the


electron


attachment


energies


required


eq 5-1


given


in Table


Before deriving the


average bond


dissoc


nation


enthalpies


for the gas


-phase M(ac


ac)3 ions


by using


5-1 (a)


and 5-1


available data


for the


corresponding neutral


bond


enthalpies


critically


assessed


SInaccurate


assumptions made


in the


literature


derivations


revise the


required

published


us to generate new experimental


enthalpies


as discussed


in the


data


and thereby


following.


For M(acac)3


complexes


average


homolytic metal-oxygen bond


enthalpy,


AhoMn (M-O),


can be


found


from the


thermochemical


cycle


Figure


5-2.


The relationship between


various


thermochemi


values


is given


5-2.


AHhom


(M-O)


= 1/6


3AH (Hacac


+ 3AHvap


(Hacac)


+ 3AHhm(H-acac)


+ AHsub


- AHf[M(acac)


- AHsub([M(acac)3


- 3/2AHf[H(g)]


5-1 (


was


are











91





O
ra
0x
i-4



of
0


0 oa


C)

2 o






OD I
0 0)'


u 0) x N
0) _____ o0o__^









*mP
o+C
U a
aa




o> 0




-m +
o 0
o a. 4-- I"
U X + Ilo

Sc 0 0__ _
10









8 0
3c ^^ ) 'U


2 eJ
,C
t a)
0
-5-..







0C *s^o^
U' 0) S

0-' -.4-
3= =










)C
4-- U..












Table


5-1.


Free


M(acac1 complexes
ions an M(Qa.


Energies


of Electron Attachment


and Enthalpies


(kcal


of Electron Attachment


mol"


to Free M+3


AGa [M(acac)3


(g) ]


AHao[M+3 bg)]


AHao[M(g) ]c


-633.53


-24.9


-675.45


-713.5


-12(

-15(


-775.9


-43.0 0.5


-47 2


-38.7


-706.35


-772.0


0.5


-656


8(4.6)


-16(5)

-25(7)


aAll


values


taken


from Table


(temperature


= 350 K)


bValues
metals


given are negative of
taken from ref. 137.


CElectron affi]
in parenthesis


na


ity data
is the u


the third
Conversion


for atomic metal


uncertainty


ionization


factor


taken


in the last


potentials


= 23.065


from ref.


Kcal


138.


of the
mol"


Number


figures)


v












between


AHho0(M-O)


and the


average


heterolytic metal


oxygen


bond


enthalpy


AHhet


given


5-3.


M-O)


The summation


- -
6


term


M-O) +


^iIP(M)
i-i


5-3 is the sum of


+3AHa


first


(acac


three


ionization


potentials


for the metal M.


Values


for AHf0


for M(acac)3


complexes


first


transition metal


series


are available


in the


literature


from


results


results


of Wood


of reaction


and Jones


using


calorimetry.


bomb calorimetry139


140-143


Reaction


from


calorimetry


considered


to be the more


reliable method


for M(acac)3


compounds


144,141a


this


technique


has been applied


to M(acac)


complexes


of interest


here


for M


= Cr, Mn,


Fe and Co.


141,142


Reaction


calorimetry


been


used by


Ribeiro


Da Silva


and co-workers


to determine values


of AHf0


AHho ( M-O


technique


for other tris


B-diketonate


to transition metal


complexes,


B-diketonates


application


has been


reviewed.


Their work

AHhno(M-O)


included


derived


a reappraisal


from the original


of of the


reaction


values


of AHf0


calorimetry


studies,


values


have


been


revised


here


using the


latest


values


of the


auxiliary thermo


chemical


data


required


for their


determination.


values


that


introduce


the greatest


uncertainty


in derived


bond


phase


enthalpies

homolytic


for the M(acac)3


bond


dissociation


complexes


enthalpy


are


values


of the O-H


of the gas-


bond


enol


form of


acetylacetone


(AHh" (H-acac)


eq 5-2)


and the enthalpy


sublimation


of the M(


acac)3


complex


Values


for AHsub0[M


acac)3]


difficult

volatility


to measure


such


prec


isely


as M(acac)3


for compounds


complexes.


of relatively


For example


values


for AHsub


fnr Crtacaci


noted


.1.. I r~ r1f~s. r rIlrrl n nfl rr~ I r*~r1 '1 aot4


--- -


CT !'A


I--- -


- I


are


(M-O


AHhet


16A hom


n1 Cob


I


I |


q


r nn u r-rn nTT nar^












earlier work


was recognized


in the reappraisal


Ribiero


Da Silva


co-workers.


From


a review of


the results


available


in the


literature,


values


of AH [M(acac)3]


chosen by these workers were


in the


range


of -28-33


kcal


1 The


same


values


were


used


in this


report


are given


in Table


5-2 along with


the other


auxiliary


data


used


in Figure


5-2.


No experimental


values


have


been available


for the


value of


AHho(H-acac),


and values


used


previously


have been


estimated.


difficulty


assess


ing the contributions


to the relative


stability


acetylacetone


due to


intramolecular


hydrogen bonding


in the


enol


form


the effects of


electronic delocalization


in the


acetylacetonate


radical


led to estimated


values


ranging


from 87-110


kcal


-1 139,1I


A value of


AHho0 (H-acac)


can be obtained


from the


gas-phase


proton


affinity


AHpA)


acac


-,146
9


acac"


and the


ioni


zation


potential


H atom.


The relationship


given by


AHhon (H-acac)


= AHpA(acac


- IP(H(g))


- AHao(acac-)


Substituting the


available


data


from the


literature


and AHa


acac"


determined


in this


report


(Table


5-2)


into eq


yields


a value


AHho(H-acac)


of 90 + 5


kcal mol


The new value


for the


AHho ( H-acac


combined


with


reaction


calorimetry


data


leads


to new values


of AHh,o(M-o)


AHhet


(M-O)


the M(


acac)


complexes


of Cr, Mn,


Fe and Co


, and these


values


are


given


in Table


5-3.


Also given


in Table


are the values


AAHhet


(acac)


obtained


from eq


the data


Tabli


resulting


AHhet


(M-O)


and AHh0 (M-O)


values


from eqs


r 4-b


af~


U -. n


S


- .


5 *% n -S a.an 1 nn~ m.. 1 a C.


5-1 (


A


ik *


[M-


C- -1


Ik J


s *". i