STEREODYNAMICS AND DEGENERATE LIGAND-EXCHANGE REACTIONS IN SOLUTIONS OF TETRACOORDINATE NON-TRANSITION METAL CHELATE COMPLEXES

Non-transition metal (d0, d10) chelate complexes possessing tetrahedral configuration at the metal centre undergo in solution fast intramolecular stereoisomerisation and intermolecular ligand exchange reactions. The inversion of stereochemical configuration is governed by two types of intramolecular interconversions: 1) polytopal rearrangement of the digonal twist type and 2) dissociation-recombination reaction due to the cleavage of one of the metal-ligand bonds. The energy preferable mechanism for the inversion is chiefly determined by the nature of a metal, but also depends on the type of ligating atoms and the more distant structural environment of the metal centre. The values of the energy barriers against the inversion of the tetrahedral configuration fall into the range of 10-30 kcal.mol-1. In the case of cadmium and heavier metal complexes, values of the activation energy of intermolecular degenerate ligand exchange reactions are comparable with those of intermolecular inversion processes. A comparison of the stereodynamics of the non-transition metal chelate complexes with the planar-tetrahedral interconversion of bis-chelate Ni(II) complexes is made.