THE INFLUENCE OF CRYSTAL PACKING ON THE MOLECULAR STRUCTURE OF MAIN GROUP ELEMENT COMPOUNDS. Edward R.T. Tiekink and Mark A. Buntine Department of Chemistry, The University of Adelaide, Adelaide, S.A. 5005, AUSTRALIA

In a number of recent reviews it has been demonstrated that the molecular structure found in main group element compounds may be dependent on seemingly minor changes in chemistry [1]. Hence, different coordination geometries, coordination numbers or even stoichiometries may be found for compounds with very similar chemical formulae [1]. For example, consider the structures of Hg(S₂COR)₂ [2]: when R = Me, a three coordinate T-shaped geometry is found for Hg in a linear polymeric array; for R = nPr, the Hg atoms are tetrahedral and the structure is comprised of two-dimensional sheets; when R = iPr, the Hg atoms are again tetrahedral, however, the structure is now a three-dimensional polymer. In the absence of obvious steric and/or electronic effects, it may be concluded that this phenomenon occurs as a result of crystal packing effects. Polymorphs provide an ideal opportunity to examine 'crystal packing' effects on molecular structure. Several organotin systems, including Ph₂Sn(bipy)Cl₂ and Ph₂Sn(S₂CNEt₂)₂, which are known to crystallise as polymorphs have been subjected to ab initio molecular orbital calculations to investigate their preferred 'gas phase' geometries. Geometry optimisations at the Hartree Fock level of theory, using both the 3-21G and LanL2DZ basis sets, have shown that the distinct structural configurations exhibited by the crystalline polymorphs independently relax to an identical gas-phase geometry. It is important to recognise that the use of two completely different types of basis set to describe these large organotin systems minimises errors associated with basis set-specific artifacts. The 3-21G basis set is a split valence descriptor, while the LanL2DZ basis set utilises a double zeta description for first row elements and an effective core potential (ECP) for heavier atoms. Calculated gas-phase stabilisation energies range from 400-1200 kJ mol^-1, depending on the system under study. More significantly, predicted stabilisation energies from each basis set description agree to within, at worst, 40 kJ mol^-1. Detailed results of this study will be presented.

[1] E.R.T. Tiekink: Main Group Chemistry News, 3, 1995, 12 - 16; Appl. Organomet. Chem. 5, 1991, 1 - 21; Main Group Met. Chemistry, 15, 1992, 161 - 186; E.R.T. Tiekink and G. Winter: Rev. Inorg. Chem., 12, 1992, 183 - 302.