Lecture 34 MO’s of the most important polyatomic ligands 1) Bonding in carbene complexes Transition metal carbene complexes are formed by carbenes, CX 2, X = H, Alk, Ar, Hlg, OR, NR 2 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 CH 2 can be readily obtained from those of H 2 O There are two types of carbene bonding to a metal:

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=CR 2. Such carbenes behave as nucleophiles: X = NR 2, 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:

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-CX 2 (the carbene is a neutral two-electron donor).

4) Phosphine complexes. MO diagram for phosphine PH 3 Phosphine complexes are widely used in coordination chemistry. Frontier orbitals of the simplest phosphine PH 3 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, eV. C 3v A1A1 zx 2 +y 2, z 2 A2A2 E(x,y)

5) Bonding in phosphine complexes Phosphine ligands can be not only good  -donors but sometime excellent  -acceptors (PF 3 form stable complexes ML 4 with Pd 0 and Pt 0 while CO does not) Frontier orbitals of some PX 3 HOMO, eVLUMO, eV PMe PH PF

6) Ligand  -acceptor properties and M-L orbital overlap LUMO’s of CO (4.6 eV), CH 2 =CH 2 (5.0 eV) and PX 3 listed above ( 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, E int, 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 E int will be a function of S.

7) Dihydrogen and alkane  -complexes. Agostic interactions Can a species with neither lone pairs (CO, PR 3 ) 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 H 2 to serve as a 2e-donor is given below. H 3 + is a know species that forms from H 2 and H + :

8) Dihydrogen and alkane  -complexes. Agostic interactions While H 2 complexes can be isolated, stable alkane complexes are unknown. Nevertheless, stable complexes with agostic (from Greek “to hold onto oneself”) CH bond are multiple.