The role of Coulombic interactions in explaining the anomalous structure of pentaphenylantimony

The observation of anomalous square-pyramidal geometry for pentaphenylantimony, (C₆H₅)₅Sb, has long been rationalized as a packing effect. The analogous P and As molecules are trigonal-bipyramidal in the solid state as expected. In this work calculations have been made of the energies resulting from intra- and intermolecular nonbonded interactions in both observed and hypothetical crystal structures of square-pyramidal and trigonal-bipyramidal pentaphenylantimony, pentaphenylarsenic, and pentaphenylphosphorus in order to determine if the solid state does indeed stabilize a non-equilibrium molecular geometry of (C₆H₅)₅Sb, and if so, how much energy is involved. The energies have been taken as pairwise sums over nonbonded atoms; the atom-atom potential functions used include repulsive, van der Waals, and Coulombic terms. Results of these calculations have been compared with work in which the potential functions did not include such 1/r terms. Both observed and hypothetical structures of all three molecules lie in true minima of the energy surface with respect to variations in cell constants and molecular position and orientation whether or not electrostatic terms are included. However, only the potential functions with Coulombic terms predict the crystallization of pentaphenylantimony as a square pyramid. The lattice-energy stabilization is then calculated to be about 15 kJ mol^-1