The role of crystal packing forces in the structure of pentaphenylantimony

Pentaphenylantimony, (C₆H₅)₅Sb, possesses square-pyramidal, rather than the expected trigonal-bipyramidal geometry in the solid state; an attempt has been made to determine whether the observed conformation results from crystal packing forces. A semi-empirical strain-energy minimization procedure has been used in which the energy is taken as a sum of bond and angle deformation terms and of repulsive and van der Waals interactions between non-bonded atoms. The phenyl rings were treated as rigid bodies having D_6h symmetry. Calculations of intramolecular and lattice energies were made for (C₆H₅)₅Sb as observed in the crystal and for a hypothetical trigonal-bipyramidal molecule and crystal structure derived from data on (C₆H₅)₅P and (C₆H₅)₅As. Comparisons show that intermolecular forces favor the square-pyramidal crystal structure by 4.6 kJ/mole but that intramolecular interactions favor trigonal-bipyramidal molecular geometry by 30.5 kJ/mole. We suggest that the failure to predict the experimentally observed structure may result from the neglect of Coulombic interactions.