General molecular mechanics approach to transition metal complexes

Transition metal complexes have been treated by a molecular mechanics approach using an MM2-like valence force field for the ligands and a Urey–Bradley force field about the metal. Metal–ligand atom bonds are parameterized with dependence on the metal covalent radius. The current metal–ligand distances and covalent radii were determined by a Simplex minimization and visual optimization of the deviations in bond distance between 230 structures from the Cambridge Crystallographic Structure Database and the force field calculated values. Moderately good reproducibility of structures is obtained with a variety of metal types with different oxidation states, and degree of unsaturation and with square-planar compounds. The current model also includes coordination of π ligands to the metal center with a 1/r6 attractive term whose minimum is 25 kcal/mol per p orbital at a distance dependent on the covalent radius of the metal and an angular attentuation factor.