AB-INITIO COMPARISON OF THE (MX(2))(2) DIMERS (M=ZN, CD, HG X=F, CL, H) AND STUDY OF RELATIVISTIC EFFECTS IN CRYSTALLINE HGF2

Structures and binding energies of the dimers (MX(2))(2) (M = Zn, Cd; X = F, Cl, H) have been computed at the ab initio pseudopotential MP2 and HF levels and compared to recent theoretical data for the correspo nding mercury species. Except for Cd2H4, all ZnX(2) and CdX(2) dimers exhibit symmetrically bridged D-2h structures. This agrees with nonrel ativistic but not with quasirelativistic pseudopotential calculations for (HgHal(2))(2) as the latter predict loose C-2h complexes of almost linear monomers. Thus, relativistic effects are responsible for the d iscontinuities in the structural preferences down group 12. Similarly, the relativistic reduction of the HgX(2) dimerization energies change s the trends in the dimerization energies from Hg > Cd > Zn to Cd > Zn >> Hg. Periodic Hartree-Fock calculations on crystalline MF(2) (M = C d, Hg) with quasirelativistic and nonrelativistic pseudopotentials con firm that relativistic effects influence the bulk properties of HgX(2) compounds exactly as suggested by the molecular model studies. In par ticular, relativity reduces the sublimation energy and probably also t he boiling and melting points of HgF2 considerably, in spite of a simu ltaneous relativistic reduction of the lattice constant. The structura l and energetic trends of group 12 compounds with electronegative elem ents may be understood on the basis of electrostatic bonding contribut ions and the reduction of these by relativity for mercury.