Oxidation state and the chemical bond in metal-organic complexes

Abstract

Results from studies of transition metal complexes with both the semi-empirical Peel method and an ab initio SCF method are discussed. It is questioned whether the concept of the oxidation state of the metal remains meaningful in the complex. Evaluation of the electronic charge distribution by population analysis of the ab initio results shows that contributions from 3d, 4s and 4p diffuse functions of the metal basis are not localized to the metal but rather to the surrounding part of the molecule. After removal of these diffuse parts both the charge and the 3d population are found to obtain values in line with those of the oxidation states of free metal ions. The most clear distinction between two oxidation states is, however, given by the d-orbital energy values. Bonding in the complexes is supported by charge transfer, σ donation from ligand-forming heteroatoms in all the complexes studied and π back donation only in strong complexes. The pictures obtained from the ab initio SCF method and the Peel method are similar.