MOLECULAR MECHANICS FOR COORDINATION-COMPLEXES - THE IMPACT OF ADDING D-ELECTRON STABILIZATION ENERGIES

An electronic energy term for transition metals has been constructed for extending Molecular Mechanics (MM) to open-shell, Werner-type coordination compounds. The d-orbital energies from a generalized ligand field calculation are used to compute the Cellular Ligand Field Stabilization Energy (CLFSE). The CLFSE models the geometrical effects of the stereochemical activity of d electrons and can be computed for any coordination number, molecular symmetry, and ligand type. In conjunction with ligand-ligand nonbonding and metal-ligand bond stretch terms, CLFSEs provide a general framework for incorporating transition metals into MM. An explicit angle-bend term is not required. After describing the theoretical basis of CLFSEs, the method is illustrated using a range of six-coordinate high-spin and four-coordinate low-spin Ni-II amine complexes plus four-, five- and six-coordinate Cu-II amine systems. For the nickel complexes, the spin-state change is modeled simply by changing the d-orbital occupancies. A single set of force field and CLF parameters simultaneously reproduces the metal coordination for all ten nickel complexes with overall root-mean-square errors of 0.010 Angstrom in Ni-N bond lengths and 0.621 degrees in N-Ni-N angles. For the copper compounds, the (slightly modified) force field automatically models the Jahn-Teller distorted structures of six-coordinate species, the planar coordination in the four-coordinate compounds, and the distorted geometries of five-coordinate systems. The root-mean-square errors in bond lengths and angles for all 15 Cu molecules are higher (0.024 Angstrom and 0.897 degrees, respectively) due to the inherent variability of the structural data. Copper complexes, especially pentacoordinate ones, are intrinsically flexible or ''plastic'' which, with the added influence of the Jahn-Teller effect, can result in large geometrical changes from relatively minor crystal packing effects.