SHAPES EMPIRICAL FORCE-FIELD - NEW TREATMENT OF ANGULAR POTENTIALS AND ITS APPLICATION TO SQUARE-PLANAR TRANSITION-METAL COMPLEXES

The application of empirical force field methods to transition-metal complexes is limited by the difficulty of describing complex molecular shapes with valence force field methods and by the lack of tested empirical parameters. The SHAPES force field is a new treatment of angular distortion (three body) terms that is based on angular overlap considerations. Angular potential energies are expressed as periodic Fourier terms in spherical internal coordinates. This formulation provides a general description of many idealized geometries including the trigonal-bipyramid, square-plane, octahedron, square-pyramid, and other geometries. Three approaches to the parametrization of the SHAPES force field (normal coordinate analysis, ab initio calculation, and structure-based optimization) are presented and are shown to give similar results. Application of the SHAPES force field to a variety of square-planar rhodium complexes demonstrates that empirical force field methods can be used to estimate the structures of these complexes with impressive accuracy (bond length rmsd = +/- 0.02 angstrom, bond angle rmsd = +/- 3-degrees.