Ligands and electron counting in organometallic chemistry

Textbook: Chapters 1.4 – 1.6, 3.3

Ligands in organometallic chemistry

2

Neutral 2e donors: PR3 (phosphines), CO (carbonyl), R2C=CR2 (alkenes), RC≡CR (alkynes, can also donate 4e), N≡CR (nitriles)

Anionic 2e donors: X- (halide), CH3- (methyl), CR3

- (alkyl), Ph- (phenyl), H- (hydride) The following can also donate 4e if needed, but initially count them as 2e donors (unless they are acting as bridging ligands): OR- (alkoxide), SR- (thiolate), NR2

- (inorganic amide), PR2- (phosphide)

Anionic 4e donors: C3H5- (allyl), O2- (oxide), S2- (sulfide), NR2-

(imide), CR22- (alkylidene)

and from the previous list: OR- (alkoxide), SR- (thiolate), NR2-

(inorganic amide), PR2- (phosphide)

Anionic 6e donors: Cp- (cyclopentadienyl), O2- (oxide) Z ligands: do not bring e to the metal: BR3, AlR3

Nomenclature

x

5-Cp 3-Cp 3-allyl 1-allyl

M

M PPh2 PPh2 1-dppe / 1-dppex

x

3

- bridging ligand

Ordering: from ACS publications In formulas with Cp (cyclopentadienyl) ligands, the Cp usually comes

first, followed by the metal center: Cp2TiCl2 Other anionic multi-electron donating ligands are also often listed in front

of the metal.

In formulas with hydride ligands, the hydride is sometimes listed first. Rule # 1, however, takes precedence over this rule: HRh(CO)(PPh3)2 and

Cp2TiH2

Bridging ligands are usually placed next to the metals in question, then followed by the other ligands Note that rules 1 & 2 take precedence: Co2(-CO)2(CO)6, Rh2(-

Cl)2(CO)4, Cp2Fe2(-CO)2(CO)2

4

Coordination geometriesCN Geometry Example

2

3, trigonal

3, T shape

4, tetrahedron

4, square planar

[NC–Ag–CN]–

Pt(PPh3)3

[Rh(PPh3)3]+

Ti(CH2Ph)4

5

M LL

L ML

L

L M

L

L

L

M

LL

L

L

ML L

L

Pt

Cl

ClCl

H

H

H

H

Coordination geometriesCN Geometry Example

5, trigonal bipyramid

5, square pyramid

6, octahedron

6, pseudo-octahedron

[Co(CNPh)5]2+

W(CO)6

FeCp2

6

L ML

L

L

L

axial

equator ial Mes TaMes

Mes

Cl

Cl

ML

L L

L

L apical

basal

ML

L L

L

L

L

M

LL

L

Coordination geometries

CN Geometry Example

6, antiprism

7, capped octahedron

7, pentagonal biprism

WMe6

[ReH(PR3)3(MeCN)3]+

[IrH5(PPh3)2]

7

ML

LL

L

L

L

ML

L L

L

L

L

L

ML

L L

L

L

L

L

Electron counting and the 18 electron rule Determine the oxidation state of the transition metal center(s) and the metal centers

resulting d-electron count. To do this one must: a) note any overall charge on the metal complex b) know the charges of the ligands bound to the metal

center (ionic ligand method) c) know the number of electrons being donated to the metal

center from each ligand (ionic ligand method)

Add up the electron counts for the metal center and ligands.

Complexes with 18e counts are referred to as saturated, because there are no empty low-lying orbitals to which another incoming ligand can coordinate. Complexes with counts lower than 18e are called unsaturated and can electronically bind additional ligands.

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A simple example

9

ReR3P CO

PR3

CH3

CO

Method

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1) There is no overall charge on the complex.

2) There is one anionic ligand (CH3-, methyl group).

3) Since there is no overall charge on the complex (it is neutral), and since there is one anionic ligand present, the Re metal atom must have a +1 charge to compensate for the one negatively charged ligand. The +1 charge on the metal is also its oxidation state. So the Re is in the +1 oxidation state.

More examples

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1.

2.

MH2C

R2P

H2C M

MH2C

R2P

H2C M

C

R2P

C

HH

H

H

C

R2P

C

HH

H

H

C

R2P

C

HH

H

H

+2e-

Metal-metal bonded example

12

MoPR2

R2P Cl

C

C

MoCl

R2P

PR2

C

C

O O

OO

Method for metal-metal complexes

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The simple rule for M-M bonding is that if you have two metal atoms next to one another and each has an odd electron-count, you pair the odd electrons to make a M-M bond.

This example also has -Cl ligands. Bridging ligands with at least 2 lone pairs almost always donate 2e- to each metal center.

Oxidation state determination: There is a total of two anionic ligands for two metal centers (overall complex is neutral). Thus each metal center needs to have a +1 oxidation state to balance the anionic ligands.

Exceptions to the 18 electron rule

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d3 d4 d5 d6 d7 d8 d9 d10 d10s1

21

ScScandium

22

TiTitanium

23

VVanadium

24

CrChromium

25

MnManganese

26

FeIron

27

CoCobalt

28

NiNickel

29

CuCopper

39

YYttrium

40

ZrZirconium

41

NbNiobium

42

MoMolybdenum

43

TcTechnetium

44

RuRuthenum

45

RhRhodium

46

PdPalladium

47

AgSilver

57

LaLanthanum

72

HfHafnium

73

TaTantalum

74

WTungsten

75

ReRhenium

76

OsOsmium

77

IrIridium

78

PtPlatinum

79

AuGold

Group 8 Metals

Early TransitionMetals

16e and sub-16e configurations are common

Coordination geometries

higher than 6

Middle TransitionMetals

18e configurations are common

Coordination geometries

of 6 are common

Late TransitionMetals

16e and sub-16e configurations are common

Coordination geometries

of 5 or lower

Different metals: general properties From left to right, the

electronegativity increases substantially: Early TM are electropositive:

often found in the highest permissible oxidation state

d2 are very easily oxidized: very basic

Late TM are relatively electronegative: Often found in low oxidation

states Back donation is not so marked:

ligands are subject to nucleophilic attack

Electronegativity

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Types of metal-ligand interactions Sigma () donor ligands Pi () donor ligands

Pi () acceptor ligands

Sigma donor Pi donor* Pi acceptor*

CR3-

H-

RO-, R2N-

F-, Cl-

RCOO-

CO, olefin

CN-

PR3

These ligands also act as donors.

Examples of donor and acceptor ligands

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d

M LComplex

M L

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d

M LComplex

M L

fullempty

d

M LComplex

M L

fullempty