TRANSITION-METAL CARBONYL CLUSTERS - A MOLECULAR MECHANICS APPROACH TO LIGAND STEREOCHEMISTRY

The surface force field for molecular mechanics simulation of the ligand structure in transition metal carbonyl clusters, originally developed by Lauher, is redesigned and implemented in the common MM2 Allinger's program. The equal potential surface (EPS) for a cluster is built by patchwork using patches whose shape depends on Crabtree and Lavin's reaction path for the terminal/mu-2-bridging/terminal interconversion. The CO ligands can float on the EPS even in the presence of a clear connectivity pattern (necessary for the energy minimization within the MM2 scheme) because their connectivity is periodically redetermined. A CO ligand is assumed to be locally connected to the metals used to generate the patches to which the ligand belongs. The program is a powerful modeler and can be used as a source of sterically reasonable geometries. The dominant contribution to the computed steric energies arises from the nonbonded interactions; hence, the comparison of modeled and experimental structures should lead to the recognition of other forces at work. Consideration is given to the case of octahedral metal carbonyl clusters with stoichiometries ranging from M6(CO)12 to M6(CO)20, and since "real" structures are only occasionally found in the global minimum of the "steric" potential energy surface, it would appear that intramolecular steric interactions are not the leading term in determining the metal carbonyl cluster stereogeometries in the solid state. It is only the interplay between many different factors (inter- and intramolecular steric interactions, charge and local bookkeeping equilization, and more specific electronic effects) that determines the real structure. Steric energies are properly used only to justify small distortions around a given geometry or to exclude a particularly crowded stereoisomer rather than to foresee the correct one.