The ligand field molecular mechanics model and the stereoelectronic effects of d and s electrons

This review discusses modifications and extensions of Molecular Mechanics which are designed to model the electronic effects of the valence d and s electrons in transition metal compounds. These effects lead to severe distortions away from the ideal geometries predicted by simple VSEPR theory. The stereochemical activity of d electrons manifests in a range of structural distortions of ionic coordination complexes typified by the Jahn-Teller elongations of six-coordinate d(9) Cu2+ species. Modelling these effects requires an additional term in the strain energy which describes the attendant ligand field stabilisation energy (LFSE). The LFSE is explicitly incorporated into the ligand field molecular mechanics (LFMM) method which has been applied to a range of complexes of Cu2+, Ni2+ and Co3+. A single set of LFMM parameters for a given metal-ligand interaction is able to model different coordination numbers, spin states and bond lengths. The stereochemical activity of the valence metal s orbital is significant for covalent organometallic species such as WMe6 which does not show the regular octahedral geometry expected for a formally d(0) system. The effect can be treated within Valence Bond theory by modifying the expressions for the angle bending potentials based on Pauling's strength functions for sd(n) hybrids. There is more than one idealised bond angle for sd(3), sd(4) and sd(5) hybrids which correlates with the irregular geometries found for hydride, alkyl and aryl compounds. The same behaviour can also be obtained within the LFMM scheme by using an extended stabilisation energy term which incorporates the s orbital contributions. The LFMM model also predicts the ligand field contribution to the activation energy for ligand exchange/substitution and can be used to calculate the structures and energies of transition states as illustrated by model calculations for the reactions of low-spin d(8) complexes. (C) 2001 Elsevier Science B.V. All rights reserved.