Valence bond concepts applied to the molecular mechanics description of molecular shapes. 4. Transition metals with pi-bonds.

We have developed a model for understanding the shapes of transition metal complexes containing multiple bonds. This model, which focuses on Lewis-like structures and the balance of forces arising from sigma- and pi-bond frameworks, provides a simple method for predicting the structures of transition metal complexes with pi-bonds. Potential energy expressions suitable for implementation in molecular mechanics computations have been derived from consideration of orbital hybridizations and coded into our UFF2-based molecular mechanics program, VALBOND. The VALBOND method correctly predicts the structures for a wide variety of experimentally and computationally characterized compounds containing metal-ligand multiple bonds.