The role of ir-bonding ligands

Octahedral metal complexes of d , Cr(III) and low spin d , Co(III), have particularly high kinetic stability to ligand replacement or exchange. The activation energy for ligand substitution in such complexes has been shown to be considerably greater than in comparable complexes in which the metal possesses other d° configurations. This kinetic stability is associated with the half filled or filled t2g environments, in which the 

Important examples of octahedral and complexes are Vitamin B 2 and certain alkyl and aryl derivatives of Cr(III) and Co(III) (see p 243). (b) Stabilization by steric effects. There is a number of thermally quite stable compounds which do not obey the 18-electron rule, for the stability of which explanations based on steric effects may be advanced. It is probable that the paramagnetic, monomeric V(CO)5 does not dimerise because this would require a co-ordination about the vanadium of seven (or more), one of the ligands being the large V(CO)6 group. 

The ethylene-metal system provides a useful model by which to describe the bonding in w-bonded metal-organic complexes. As we shall see, such bonding is difficult to treat in terms of a conventional classical valence bond approach. It is more meaningful to use descriptions based on molecular orbital theory. 

Since only those ligand and metal orbitals which have the same symmetry properties can combine to form bonding M.O.s, we may by symmetry considerations determine the often fairly small number of combinations of ligand and metal orbitals which can lead to bond formation. 

It should be noted that we are restricting ourselves to the valence atomic orbitals of the metal and the /m-M.O.s of the ligand. 

---