MS-X-ALPHA STUDIES ON THE COLORS OF BIPH5, PBCL6(2-), AND WS4(2-) - ARE RELATIVISTIC EFFECTS ON THE LUMO IMPORTANT

Quasirelativistic multiple-scattering X-alpha (MS X-alpha) and pseudopotential Hartree-Fock calculations are performed on MX5 (M = Bi, Sb; X = H, CCH) model systems in D3h or C4v site symmetries to elucidate the origin of the violet color of pentaphenylbismuth. The occurrence of this color only for Bi and for C4v symmetry is related to the relativistic stabilization of the a1 LUMO as concluded earlier from REX calculations. The nonrelativistic transitions are symmetry-forbidden. The relativistic perpendicular C4v transition is allowed, in agreement with experiment. The study is extended further to include MS calculations on octahedral and tetrahedral site symmetries. The yellow color of PbCl6(2-) is also attributed to the relativistic stabilization of the a1g LUMO. The color difference between MoS4(2-) and WS4(2-) is mainly due to the shell-structure effects and not relativity.