organometallic chemistry an overview of structures and reactions peter h.m. budzelaar
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Organometallic Chemistry
an overview of structures and reactions
Peter H.M. Budzelaar
Organometallic Chemistry2
Between organic and inorganic...
Organic chemistry:• more or less covalent C-X bonds• rigid element environments• fixed oxidation states (better: valencies)
??Organometallic chemistry??
Inorganic chemistry:• primarily ionic M-X bonds, dative M-L bonds• variable and often fluxional environments• variable oxidation states
Organometallic Chemistry3
Organometallic reactivity
Since most organometallics are intermediates, the focus in organometallic chemistry is usually on understanding and tuning reactivity
This starts with analyzing reaction mechanisms in terms of elementary steps
The number of possible elementary steps is larger than in"pure organic" chemistry, but the ideas are similar
Organometallic Chemistry4
Organometallic structures
Knowledge of inorganic and coordination chemistry is useful to understand geometries, electron counts and oxidation states of organometallic compounds
Organometallics are more covalent and often less symmetricthan coordination compounds, so orbital symmetry argumentsare not as important
Organometallic Chemistry5
Trends in organometallic chemistry
Organometallic chemistry is concerned with all metals, in combination with all "organic" elements.
there are many metals !
Generalization is importantthe chemistry of e.g. Fe is not much more complicated
than that of C, but after that there are 80 more metals...
we divide reactions in broader categories than organic chemists do
We concentrate on the M side of the M-C bond,and on how to tune its reactivity
Organometallic Chemistry6
Elements of interest
Organic elementsMain group metalsTransition metals
Organometallic Chemistry7
Organic vs organometallic reactivity
Organic chemistry:C-C / C-H : nearly covalent
C+-X- : polar (partly ionic)
reactivity dominated by nucleophilic attack at C
SN2 and SN1 like reactivity
Organometallic chemistry:C is the negative end of the M-C bond ("umpolung")
reactivity dominated by electrophilic attack at Cor nucleophilic attack at M
associative and dissociative substitution at M
X
SN2 TS (10-e ?)
Y
- X Y
- XY
X
Y
SN1 int (6-e)
(C2H4)2PdCl2 (C2H4)(CO)PdCl2
(C2H4)PdCl2
(C2H4)2(CO)PdCl2
?
CO- C2H4
- C2H4CO
dissociative
associative
Organometallic Chemistry8
Main-group organometallics
s and p orbitals.8-e rule, usually.
with a lot of exceptions
More electropositive and larger:higher coordination numbers,regardless of the number of electrons.
“Early" groups and not very electropositive:lower coordination numbers.
Al
Me
Al
MeMe
MeMe
Me
N
Cl
Sn
OMe
ClMe
Me Zn Me
8-e
10-e
4-e
Organometallic Chemistry9
Main-group organometallics
Metal is the "+" side of the M-C bond.
Chemistry dominated by nucleophilic attack of C- at electrophiles.this is also the main application in organic synthesis
MeMgBr + Me2CO Me3COMgBr
note: this is a simplified pictureof the Grignard reaction
Organometallic Chemistry10
Main-group organometallics
M-M multiple bonds are fairly weak and rather reactivethey are a curiosity and relatively unimportant,
certainly compared to C-C multiple bonds
Bond strengths in kcal/mol:
C-C 85 C=C 150
N-N 40 N=N 100
P-P 50 P=P 75
Multiple-bonded compounds often have unusual geometries
Me4C2
"Me4Sn2"
Organometallic Chemistry11
Transition-metal organometallics
s, p and d orbitals
18-e rule, sometimes 16-eother counts relatively rare
CO
Cr
CO
OC CO
COOCFe
Ni
18-e
18-e
18-e
PtEt3P Me
ClEt3P
16-e
Organometallic Chemistry12
Transition-metal organometallics
Lower electron counts if metals are sterically saturated:
nor
Co
nornor
nor
nor =
13-e
Me
W
Me
Me Me
MeMe
12-e
note: actual structure is not octahedral !
TiMeMe
16-e
Organometallic Chemistry13
Transition-metal organometallics
Often ligands capable of donating 2-8 electronsPreference for -system ligands (good overlap with d-orbitals)
Bonding to neutral ligands (olefin/diene/CO/phosphine)relatively weak
Important for catalysis!
allyl
3 e
cyclopentadienyl
5 e
O
C
2 e
C
carbene
2 e
olefin
2 e
Organometallic Chemistry14
Transition-metal organometallics
“Forbidden” reactions ?
+?
Symmetry-forbiddenDoes not happen
Mo Mo+
No symmetryVery fast
a
Organometallic Chemistry15
Reactivity of the M-C bond
Polar reactive towards e.g.
• Water:Me3Al explodes with water; Me4Sn does not react.
• Oxygen:Me2Zn inflames in air; Me4Ge does not react.
• Carbonyl groups:MeLi adds at -80°C, Me3Sb not even at +50°C.
M-OH + H-C
M-O-O-C
M-O-C C
Organometallic Chemistry16
Reactivity of the M-C bond
Oxidation and hydrolysis: large driving force
Bond strengths in kcal/mol:
Al-C 65 As-C 55 Si-C 74
Al-O 119 As-O 72 Si-O 108
Al-Cl 100 Si-Cl 91
Organometallic Chemistry17
Organometallic reaction steps
Ligand dissociation / coordination
Me3Al + NMe3 Me3Al-NMe3
6e 8e
Mo(CO)6 Mo(CO)5 + CO
18e 16e
note: free Me3Aldimerizes to Me6Al2
Organometallic Chemistry18
Organometallic reaction steps
Insertion and -elimination
MeMgBr + MeC N Me2C=NMgBr
FeN
NN Me Fe
NN
N Me FeN
NN
Me
N
N NFeAr Ar
+
Ar =
FeN
NN Me =
Organometallic Chemistry19
Organometallic reaction steps
Insertion and -eliminationH
PdP
PPdP
P
H
Ph2P PPh2=PP
Organometallic Chemistry20
Organometallic reaction steps
Oxidative addition / Reductive elimination
PRh
P
Cl
P
PRh
P
H
P
H
Cl
H2"P" = PPh3
Organometallic Chemistry21
Organometallic reaction steps
Oxidative addition / Reductive eliminationP
RhO
O
P
MeI PRh
O
O
P
Me
I
Ph2PPPh2=PP
Organometallic Chemistry22
Organometallic reaction steps
-bond metathesis
14 e
Lu CH3 Lu *CH3*CH4
LuC
*CH
H3
H3
Organometallic Chemistry23
Organometallic reaction steps
Redox reactions
Homolysis
FeFe- e-
Et Hg Et Et Hg Et+
Organometallic Chemistry24
Organometallic reaction steps
Reactivity of coordinated ligands
Organometallic Chemistry25
Factors governing structure and reactivityof organometallic compounds
• M-C, M-X bond strengths
• Electronegativity of M (polarity of M-C etc bonds)
• Number of (d) electrons
• Coordination number
• Steric hindrance
Organometallic Chemistry26
Trends in the periodic table
Main group metals:• left and down: more electropositive• down: higher oxidation states less stable
Transition metals:• middle: strongest preference for 18-e• 2nd and 3rd row: strong preference for
paired electrons (low-spin states)• down: higher oxidation states more stable
Organometallic Chemistry27
Working with organometallics
Synthesis and reactivity studies (inert atmosphere!):• Glove box• Schlenk line, specialized glassware
Characterization:• Xray diffraction structure bonding• NMR structure en dynamic behaviour• (calculations)
• IR• MS• EPR
Not:• GC• LC