Lattice energy calculations for (C₆H₅)₅M0.5C₆H₁₂, M = P, As and Sb: towards an understanding of crystal packing in the pentaphenyl group V compounds

Semi-empirical calculations of the energy due to intermolecular interactions have been made for crystals of (C₆H₅)₅M. ½C₆H₁₂, M= P, As, and Sb. This energy has been calculated as a pairwise sum over nonbonded atoms; energy minimizations have been performed with respect to the unit-cell parameters and molecular positions and orientations. Two sets of potential functions including repulsive and van der Waals terms have been employed; one of the sets also contains r^-1 (Coulombic) contributions to the energy. The experimentally determined structures of the arsenic and antimony compounds have been well reproduced, as has the disorder observed for the solvent molecule in the former; it is suggested that this disorder is also present in the latter crystal. A comparison of the energies calculated for these solvates with those computed previously for unsolvated species predicts the stability of (C₆H₅)₅P.½C₆H₁₂, for which no crystallographic data have been previously reported. Crystals of this compound have now been identified. These comparisons also show the relative lattice energies of the three possible crystal structures seen in this system (Cc, P, and the P cyclohexane solvate considered here) to be the same for all three molecules, suggesting that the molecular packing is not greatly affected by a change in the central group V atom.