MOLECULAR MECHANICS FOR ORGANOMETALLIC MOLECULES: PREDICTION AND EXPLANATION. Tatjana V. Timofeeva and Norman L. Allinger, Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov St., 28, 117813, Russia Department of Chemistry, Computational Center for Molecular Structure and Design, University of Georgia, Athens, Georgia 30602-2556, USA

In recent years molecular mechanics (MM) begun to penetrate the wide field of organometallic chemistry. Until recently only few attempts have been made to calculate such molecules. The calculations were hindered by two problems, first, the connectivities (graphs) of these molecules are not compartable with traditional MM software. The second reason is that too many different types of bond angles have to be described for the same atom type. It is well known that that ligands in organometallic molecules have no consistent bond angles to describe their positions in the coordination sphere of the metal atom. So very often the leading role in formation of coordination sphere around metal atom plays ligand-ligand non-bonded interactions. To describe ligand positions two ways could be used: dummy atom technique and quasi non-bonded interactions of molecular fragments. For instance any metallocene molecule could be described as metal atom joint by two bonds to the centers of the Cp-rings (dummy atoms), or like metal atom joint with 10 carbon atoms of Cp-rings by strong potential of non-bonded type (6-exp, 6-12).

Using mentioned approach several series of organometallic molecules have been described in a free state and in crystal. It was shown that for unsubstituted and substututed metallocenes of main II and IVA groups and for lantanides II the conformation of bent sandwich is preferable in free and in solid state. It is due to the interligand attraction when M-ligand distances are long. The bending is bigger when the metal-carbon distances (metal radii) is bigger. Systematical increase of bending in crystal according to our calculations is due to additional M-ligand interaction with the ligands of the neighboring molecules. The same regularity was found for a series of sandwich metallocarboranes of the [RR'C₂B₄H₂]₂M type, where M=Si, Ge, Sn, Pb. Molecules with short M-ligand distances (M=Si, Ge) have parallel, and bent-sandwich conformation for atoms with bigger radii (M=Sn, Pb).