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

This review discusses modifications and extensions of Molecular Mechanics w hich are designed to model the electronic effects of the valence d and s el ectrons in transition metal compounds. These effects lead to severe distort ions away from the ideal geometries predicted by simple VSEPR theory. The s tereochemical activity of d electrons manifests in a range of structural di stortions of ionic coordination complexes typified by the Jahn-Teller elong ations of six-coordinate d(9) Cu2+ species. Modelling these effects require s an additional term in the strain energy which describes the attendant lig and field stabilisation energy (LFSE). The LFSE is explicitly incorporated into the ligand field molecular mechanics (LFMM) method which has been appl ied to a range of complexes of Cu2+, Ni2+ and Co3+. A single set of LFMM pa rameters for a given metal-ligand interaction is able to model different co ordination numbers, spin states and bond lengths. The stereochemical activi ty of the valence metal s orbital is significant for covalent organometalli c 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 Valen ce Bond theory by modifying the expressions for the angle bending potential s based on Pauling's strength functions for sd(n) hybrids. There is more th an one idealised bond angle for sd(3), sd(4) and sd(5) hybrids which correl ates with the irregular geometries found for hydride, alkyl and aryl compou nds. The same behaviour can also be obtained within the LFMM scheme by usin g an extended stabilisation energy term which incorporates the s orbital co ntributions. The LFMM model also predicts the ligand field contribution to the activation energy for ligand exchange/substitution and can be used to c alculate 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.