Lecture 34

MO’s of the most important polyatomic ligands 1) Bonding in carbene complexes

• Transition metal carbene complexes are formed by carbenes, CX2 , X = H, Alk, Ar, Hlg, OR, NR2 etc. These complexes are responsible for catalysis of olefin metathesis (Y. Chauvin, R. Schrock, R. Grubbs, 2005 Nobel prize for Chemistry).

Frontier orbitals of the singlet CH2 can be readily obtained from those of H2O

• There are two types of carbene bonding to a metal:

C

a1

C

b2

(-10.5 eV) (2.0 eV)

HOMO LUMO

CH2

C CM

-bonding

M

-backbonding

M C M C

2) Nucleophilic carbene complexes

• Contribution of - and -bonding into the net M-carbene bond determines chemical reactivity of a carbene complex and depends on the nature of substituents X attached to the carbene carbon.

• X = Alk or H (X is a -only donor) corresponds to the case of “nucleophilic” carbene complexes. For C(Alk)2 -bonding of the carbene carbon to a metal is important so that the carbene can be considered as a dianionic four-electron ligand attached to a metal with a double bond, M=CR2.

• Such carbenes behave as nucleophiles:

• X = NR2, OR or halogen (X is a -donor) corresponds to the case of “electrophilic” carbene complexes (“Fisher metal carbenes”). Interestingly, cyclic diaminocarbenes (Arduengo carbenes) with bulky R (i-Pr, tert-Bu, Ad) are stable in a free form:

(OC)5Cr C

OEt

NMe2

1.35 (1.41 for a single bond)

1.33 (1.45 for a single bond)

2.13

RN

C

NR

RN

C

NR

RN

C

NR

(Me3CCH2-)3Ta CHCMe3

1.942.24

O CR2

(Me3CH2C)3TaCR2

OCHCMe3

+

M C

3) Electrophilic carbene complexes

• Strong nucleophiles can attack the carbene carbon so proving that the Fisher carbene complexes are electrophilic in some degree.

• In metal-free Arduengo carbenes interaction of empty p-orbital of the carbene carbon with filled p-orbitals of the adjacent atoms raises the energy of the former. It matches the energy of filled metal orbitals only poorly now and thus is a week acceptor . So, the metal-to-carbene bonding can be better described with the formula M-CX2 (the carbene is a neutral two-electron donor).

(OC)5Cr CMe

OMe

(OC)5Cr CMe

OMeNHR

NH2R

- H+ (OC)5Cr CMe

NHR

- MeO

HNC

NH

NR

RN

C HNC

NH

HNC

NH

(6.2 eV)

(-9.5 eV)

(-13.0 eV)

C N+ 2

(-9.2 eV)

(-11.4 eV)(2.0 eV)

4) Phosphine complexes. MO diagram for phosphine PH3

• Phosphine complexes are widely used in coordination chemistry. Frontier orbitals of the simplest phosphine PH3 resemble closely those of ammonia with the difference that the LUMO is two degenerate e-orbitals. What about phosphorus d-orbitals? Their energy is too high, +23.1 eV.

3 HP

a1 (3pz)

a1 (3s)

s1 + s2 + s3

e (3px, 3py)

1a1

2a1

1e

3a1

e

a12s1 - s2 - s3

- s2 + s3

2e

(-23.1 eV)

(-14.4 eV)

(-10.3 eV)

(-22.7 eV)

(-10.6 eV)

(4.6 eV)

(7.5 eV)

x

y

z

C3v

A1 z x2+y2, z2

A2

E (x,y)

5) Bonding in phosphine complexes

• Phosphine ligands can be not only good -donors but sometime excellent -acceptors (PF3 form stable complexes ML4 with Pd0 and Pt0 while CO does not)

Frontier orbitals of some PX3

HOMO, eV LUMO, eV

PMe3 -8.8 5.5

PH3 -10.3 4.6

PF3 -12.5 4.1

M

P

dxz

P

MM

dyzdz2

P(-10.3 eV) (4.6 eV)

x

y

z

-bonding -backbonding

6) Ligand -acceptor properties and M-L orbital overlap

• LUMO’s of CO (4.6 eV), CH2=CH2 (5.0 eV) and PX3 listed above (4.1 - 5.5 eV) are close in energy. But -acceptor properties of these ligands are very different. Why?

• The energy of interaction of orbitals of the metal and the ligand, Eint, is a function of two parameters, an overlap integral S and the difference in energy of overlapping orbitals. For HOMO of a metal and LUMO of a ligand we have:

• Assuming that for one and the same metal and a series of ligands E(ligand)LUMO - E(metal)HOMO ≈ constant, we get that Eint will be a function of S.

HOMOLUMO metalEligandE

SE

)()(

2

int

M M

CO

M

PH3

M

PF3olefin

(5.0 eV) (4.6 eV) (4.6 eV) (4.1 eV)

S increases S increases

7) Dihydrogen and alkane -complexes. Agostic interactions

• Can a species with neither lone pairs (CO, PR3) nor -bonds serve as an electron donor?

• Dihydrogen, CH bonds in alkyls groups and alkanes and some other electron-rich -bonds (Si-H, B-H, etc.) can. A simple way to illustrate the ability of H2 to serve as a 2e-donor is given below. H3

+ is a know species that forms from H2 and H+:

3 (1s)

(-13.5 eV)

(-33.3 eV)

(-4.8 eV)

a1"

e'

2 H + H+

H3+

8) Dihydrogen and alkane -complexes. Agostic interactions

• While H2 complexes can be isolated, stable alkane complexes are unknown. Nevertheless, stable complexes with agostic (from Greek “to hold onto oneself”) CH bond are multiple.

WOC

PCy3

OC

CO

PCy3

H

H0.84 (0.74 in free H2)

-bonding

M

-backbonding

MH

H

H

H

(-16.3 eV) (6.6 eV)

Ti

PMe2

Me2P

Cl

Cl

HH

H

84.5o

2.10

1.13

Cl

MH H

-bonding

M

-backbonding

C C(-14.8 eV) (7.0 eV)

RhOC

H

H

M+

Ir H

PCy3

H

PCy3

H

HHH

2-C,H

2-H,H