Molecular orbitals of transition metal complexes

288 Pages·2005·11.37 MB·english

by  Yves Jean

Molecular Orbitals of Transition
Metal Complexes
Molecular Orbitals of Transition
Metal Complexes
Yves Jean
LaboratoiredeChimiePhysique,
UniversitéParis-Sud
Translatedby
ColinMarsden
LaboratoiredePhysiqueQuantique,
UniversitéPaulSabatier,
Toulouse
1
3
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Foreword
Wearesodivided. Bytheformalstructureofuniversityinstruction—
organic chemistry, inorganic chemistry, physical chemistry. By the
incredibleandunnecessaryspecializationofourjournals.Themolecu-
larbountywehaveourselvescreatedseemssimplyoverwhelming—no
wonder we seek compartmentalization in self-protection: It is easy to
say,‘I’manexpertinFieldx.AndwhileIwilllistentoaseminarinyorz
(whenIhavetime),please...letmebehappyjustinkeepingupwith
myownfield.’
Thedangersofspecializationareobvious—inbreeding,lackofscope,
a kind of rococo elaboration of chemical complexity within a field.
Andweknowthatnewideasoftencomefromanalmostmetaphorical
importationofawayofthinkingoratechniquefromanotherarea.
Meanwhile, all along, nature persists in subverting the compart-
mentalizingsimplicityofourminds.Throughenzymeswhoseseeming
magicisdonebymetalatomsandclustersattheactivesite, inorganic
chemistry and biochemistry are rejoined. Transition metal carbides
putorganiccarbonintosomemostunusual, patentlyinorganicenvir-
onments. And, beginning in 1950, the explosion in organometallic
chemistryhasgivenusanincrediblerichesofstructuresandreactions—
from ferrocene to olefin metathesis, metal–metal multiple bonds, to
C H activation, and remarkable olefin polymerization catalysts. All
−−
fromacombinationofinorganicandorganicchemistry.
Organometallic chemistry from its beginning also depended on,
and also built, another bridge. This is to theoretical chemistry. The
first, rationalizing accounts of the electronic structure of ferrocene
and the Dewar–Chatt–Duncanson picture of metal–olefin bonding
werefollowedbymilestonessuchasthepredictionofcyclobutadiene–
iron tricarbonyl and Cotton’s beautiful elaboration of the idea of a
metal–metalquadruplebond. TheworkofLeslieOrgelplayedavery
importantroleinthoseearlydays. Therewerefecundinteractionsall
along—compounds leading to calculations, and calculations pushing
experimentalists to make new molecules. Often the theory was done
bytheexperimentaliststhemselves,forthebestkindoftheory(theone
thatkeepsthefertiledanceofexperimentmoving)isaportableone.As
easytousemolecularorbitaltheory,thetheoryofchoiceofthetimes,
mostcertainlywasandis.
Foreword
It is hard to imagine a contemporary course in organometallic
chemistry which does not contain a hefty, albeit qualitative compon-
ent of molecular orbital theory. Yves Jean (with François Volatron)
earlierwroteaclassicteachingtextontheorbitalsoforganicmolecules.
Herehehasappliedhisgreatpedagogicalskillstotheconstructionof
a beautifully thought through exposition of bonding in organometal-
licchemistry.Ourundergraduateandgraduatestudentswillenjoythis
book.Andthey,thechemistsofthefuture,willusetheknowledgegained
here to enlarge our experience with new organometallic molecules,
subverting once again the arbitrary division of organic and inorganic
chemistry.Moleculeswhosebeautyandutilitywestillcannotimagine.
RoaldHoffmann

Acknowledgements
I would like to thank several colleagues who accepted to read and
re-readthemanuscriptduringitspreparation,andwhoseremarksand
comments were extremely useful to me: O. Eisenstein and C. Iung
(Montpellier 2), J. Y. Saillard (Rennes 1), A. Strich (Strasbourg 1),
I.Demachy(ParisSud,Orsay),P.LeFlochandhisgroup(D.Moores,M.
Melaimi,N.Mezailles,M.Doux)attheEcolePolytechnique(Palaiseau),
R. Hoffmann (Cornell). My thanks also go to J. Courtieu (Paris Sud,
Orsay), thanks to whom I have been able to give a lecture course on
orbitalinteractionstochemistrystudentssince1987, andtoA.Lledos
(Barcelona), who encouraged me to write this book at a time when I
washesitating.Forvariousreasons,IamgratefultoF.Mathey(Director
ofthe‘HétéroélémentsetCoordination’laboratoryattheEcolePoly-
technique),L.Salem(founderofthelaboratory‘ChimieThéorique’at
Orsay),C.Pouchan(Pau),J.C.Rayez(Bordeaux1),J.L.Rivail(Nancy1),
A.Fuchs(Directorofthe‘LaboratoiredeChimiePhysique’atOrsay),
M.andP.Jean,fortheirhelp.
Thisbookalsoowesalargedebttothestudentswhohavefollowed
my lectures. In order to transmit knowledge and understanding, even
in an area which is completely familiar, it is necessary to clarify one’s
ideassoastomakeaconsistentand,ifpossible,attractivepresentation.
Students’comments,remarks,andquestionsreallyhelptheteachersto
achievethisgoal.Whenitcomestowritingamanuscript,onerealizes
justhowusefultheslowdevelopmentandmaturationofideashasbeen.
I am very honoured that Roald Hoffmann accepted to write the
preface,andthereaderwilldiscoverduringthechaptersjusthowmuch
thebookowestohim.
EcolePolytechnique(Palaiseau)
UniversitéParisSud(Orsay)
April2004

Introduction 1
Chapter1 Settingthescene 3
1.1. Electroncountinacomplex:thecovalentmodel 4
1.1.1. Ligandclassification(LorX) 4
1.1.2. Electroncountandthe18-electronrule 8
1.2. Analternativemodel:theionicmodel 12
1.2.1. Lewisbasesasligands 12
1.2.2. Equivalenceofthecovalentand
ionicmodels:examples 14
1.3. Principlesoforbitalinteractions 16
1.3.1. Interactionbetweentwoorbitalswith
thesameenergy 16
1.3.2. Interactionbetweentwoorbitalswith
differentenergies 17
1.3.3. Theroleofsymmetry 18
1.3.4. σ andπ interactions 19
1.4. Metalorbitals 19
1.4.1. Descriptionofthevalenceorbitals 20
1.4.2. Orbitalenergies 23
1.5. Ligandorbitals 24
1.5.1. Asingleligandorbital:σ interactions 24
1.5.2. Severalorbitals:σ andπ interactions 26
1.6. InitialorbitalapproachtoML complexes 30
ℓ
1.6.1. Simplifiedinteractiondiagram 30
1.6.2. Strong-fieldandweak-fieldcomplexes 31
1.6.3. Electronicconfigurationandthe
18-electronrule 31
1.6.4. Analogywiththeoctetrule 32
Exercises 33
Chapter2 Principalligandfields:σ interactions 37
2.1. OctahedralML complexes 38
6
2.1.1. Initialanalysisofthemetal–ligandorbital
interactions 38
2.1.2. Completeinteractiondiagram 41
2.1.3. Electronicstructure 48
2.2. Square-planarML complexes 51
4
2.2.1. Characterizationofthedblock 51
2.2.2. Electronicstructurefor16-electrond8
complexes 53
2.3. Square-basedpyramidalML complexes 53
5

Contents
2.3.1. Characterizationofthedblock(metalinthe
basalplane) 54
2.3.2. Characterizationofthedblock(metaloutofthe
basalplane) 56
2.3.3. Electronicstructureandgeometry 60
2.4. TetrahedralML complexes 62
4
2.4.1. Characterizationofthedblock 63
2.4.2. Electronicstructure 66
2.4.3. ML complexes:square-planaror
4
tetrahedral? 66
2.5. Trigonal-bipyramidalML complexes 69
5
2.5.1. Characterizationofthedblock 69
2.5.2. Electronicstructure 72
2.6. Trigonal-planarML complexes 73
3
2.6.1. Characterizationofthedblock 73
2.6.2. 16-electrond10complexes 74
2.7. LinearML complexes 74
2
2.7.1. Characterizationofthedblock 75
2.7.2. Electronicstructure 76
2.8. OthercomplexesorML fragments 76
n
2.8.1. PyramidalML complexes 77
3
2.8.2. ‘T-shaped’ML complexes 79
3
2.8.3. ‘Butterfly’ML complexes 81
4
2.8.4. BentML complexes 83
2
2.8.5. MLcomplexes 84
Exercises 85
AppendixA:polarizationofthedorbitals 89
AppendixB:Orbitalenergies 94
Chapter3 π-typeinteractions 97
3.1. π-donorligands:generalproperties 98
3.1.1. Thenatureoftheπ orbitalontheligand 98
3.1.2. ‘Single-face’and‘double-face’π-donors 99
3.1.3. Perturbationofthedorbitals:thegeneral
interactiondiagram 100
3.1.4. Afirstexample:theoctahedral
complex[ML Cl] 101
5
3.2. π-acceptorligands:generalproperties 104
3.2.1. Thenatureoftheπ orbitalon
theligand 104
3.2.2. ‘Single-face’and‘double-face’
π-acceptors 105
3.2.3. Perturbationofthedorbitals:thegeneral
interactiondiagram 107

Contents
3.2.4. Afirstexample:theoctahedralcomplex
[ML CO] 108
5
3.3. Complexeswithseveralπ-donoror
π-acceptorligands 111
3.3.1. Thetrans-[ML Cl ]octahedral
4 2
complex 111
3.3.2. Thetrans-[ML (CO) ]octahedral
4 2
complex 116
3.3.3. Constructionofthed-blockorbitals
‘byhand’ 117
3.3.4. MCl and M(CO) octahedral
6 6
[ ] [ ]
complexes 123
3.4. π complexes:theexampleofethylene 125
3.4.1. Orbitalinteractions:the
Dewar–Chatt–Duncansonmodel 125
3.4.2. Electronicstructureofad6complex
[ML (η2-C H )] 126
5 2 4
3.4.3. MetallocenesCp M 129
2
3.4.4. Cp ML complexes 130
2 n
3.5. π interactionsandelectroncounting 133
Exercises 135
AppendixC:Thecarbonylligand,adouble-face
π-acceptor 138
Chapter4 Applications 141
4.1. Conformationalproblems 141
4.1.1. d8- ML (η2-C H ) complexes 141
4 2 4
[ ]
4.1.2. d6- ML (η2-C H ) complexes:staggeredor
5 2 4
[ ]
eclipsedconformation? 144
4.1.3. d6- ML (η2-C H ) complexes:couplingoftwo
4 2 4 2
[ ]
π-acceptorligands 147
4.1.4. OrientationofH inthe‘Kubascomplex’
2
[W(CO) (PR ) (η2-H )] 152
3 3 2 2
4.2. ‘Abnormal’bondangles 156
4.2.1. Agosticinteractions 156
4.2.2. d6ML complexes:a‘T-shaped’or‘Y-shaped’
5
geometry? 160
4.3. Carbenecomplexes 165
4.3.1. Ambiguityintheelectroncountforcarbene
complexes 165
4.3.2. Twolimitingcases:FischercarbenesandSchrock
carbenes 166
