Deriving a mechanistic model for potential energy surface of coordination compounds of nontransition elements

Abstract

AbstractMolecules of coordination compounds (those formed by a central atom—largely by a transition or non‐transition metal ion, but also by non‐transition elements like sulfur, phosphorus, etc., and by atoms or groups surrounding it—the ligands) for decades represent a significant problem for any “classical” description given in terms of empirical force fields of molecular mechanics (MM) due to diversity of coordination polyhedra resulting in the numerosity of the parameters necessary to describe the objects of interest within such a setting. This situation is further toughened by the specific collection of effects known as mutual influence of ligands, of which the trans‐effect in transition metal complexes is the most known. A feature particularly complicating understanding the ligand influence is its qualitative dependence on the nature of the central atom. The real source of these problems is of course the specificity of the electronic structure of the coordination compounds. If compared with usual organic molecules for which the MM in its classical form is rather successful the major distinction characteristic for coordination compounds is the absence of fairly distinguishable two‐center two‐electron bonds incident to its central atom (ion). Using a methodology called deductive molecular mechanics (DMM) we have recently shown that for “organic” molecules it is possible to sequentially derive the form of the MM force field departing from a simple but intuitively transparent model of electronic structure of a single chemical bond. Here we present an analogous derivation for the force fields describing coordination compounds. It is based on the analysis of electronic structure of the closest ligand shell of coordination compounds and on that of its relation with the geometry changes induced by chemical substitution (ligand influence) performed recently by Levin and Dyachkov. By using the elements of the one‐electron density matrix as an economical set of electronic structure variables for the closest ligand shell, the DMM model of coordination compounds is constructed. Next, by excluding the electronic structure variables the effective elastic elements describing the force fields concordant with the mutual ligands' influence in coordination compounds are constructed and their dependence on the state of the central ion is established. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007