3 - overview

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Organometallic Chemistry

an overview reactions

Peter H.M. Budzelaar

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Overview of 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

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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 ofelementary steps

The number of possible elementary steps is larger than in

"pure organic" chemistry, but the ideas are similar

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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 arguments

are not as important

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Trends in organometallic chemistry

Organometallic chemistry is concerned with allmetals, in

combination with all"organic" elements.

there are manymetals !

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

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Elements of interest

Organic elements

Main group metals

Transition metals

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Organic vs organometallic reactivity

Organic chemistry:

C-C / C-H : nearly covalent

Cd+-Xd- : 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 C

or 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

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Main-group organometallics

s andp orbitals.

8-e rule, usually.

with a lotof exceptions

More electropositive and larger:

higher coordination numbers,

regardless of the number of electrons.

Early" groups and not very electropositive:

lower coordination numbers.

AlMe

Al

MeMe

MeMe

Me

N

Cl

Sn

O

MeCl Me

Me Zn Me

8-e

10-e

4-e

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Metal is the "d+" side of the M-C bond.

Chemistry dominated by nucleophilic attack of Cd- at electrophiles.

this is also the main application in organic synthesis

MeMgBr + Me2CO Me3COMgBr

note: this is a simplifiedpicture

of the Grignard reaction

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M-M multiple bonds are fairly weak and rather reactive

they 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

"Me4

Sn2

"

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Transition-metal organometallics

s,p and dorbitals

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

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Lower electron counts if metals are sterically saturated:

nor

Co

nornor

nor

nor =

13-e

12-e

TiMe

Me

16-e

Me

WMe

Me

MeMeMe

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Often ligands capable of donating 2-8 electrons

Preference forp-system ligands (good overlap with d-orbitals)

Bonding to neutral ligands (olefin/diene/CO/phosphine)

relatively weak

Important for catalysis!

allyl

3 e

cyclopentadieny

5 e

O

C

2 e

C

carbene

2 e

olefin

2 e

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An olefin complex: (Acac)Ir(NCMe)(C2H4)

donation

backdonation

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Forbidden reactions ?

+?

Symmetry-forbiddenDoes not happen

Mo Mo+

No symmetryVery fast

a

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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 -80C, Me3Sb not even at +50C.

M-OH + H-C

M-O-O-C

M-O-C C

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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 108Al-Cl 100 Si-Cl 91

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Organometallic reaction steps

Ligand dissociation / coordination

Me3Al + NMe3 Me3Al-NMe3

6e 8e

Mo(CO)6 Mo(CO)5 + CO

18e 16e

note: free Me3Al

dimerizes to Me6Al2

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Insertion and b-elimination

MeMgBr + MeC N Me2C=NMgBr

Fe

N

NN Me Fe

N

NN Me Fe

N

NN

Me

N

N NFeAr Ar

+

Ar =

FeN

NN Me =

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Insertion and b-eliminationH

PdP

PPdP

P

H

Ph2P PPh2=PP

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Oxidative addition / Reductive elimination

PRh

P

Cl

P

PRh

P

H

P

H

Cl

H2"P" = PPh3

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Oxidative addition / Reductive elimination

Rh

L

O

O

L

RhL

O

O

L

Me

I

L = P(OPh)3

RhMe

O

O

I

L

L

L = PPh3

MeI

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s-bond metathesis

14 e

Lu CH3 Lu *CH3*CH4

Lu

C

*CH

H3

H3

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Redox reactions

Homolysis

FeFe- e-

Et Hg Et Et Hg Et+

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Reactivity of coordinated ligands

F t i t t d ti it

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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

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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

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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

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