DOMINANCE OF LINEAR-2 COORDINATION IN MERCURY CHEMISTRY - QUASI-RELATIVISTIC AND NONRELATIVISTIC AB-INITIO PSEUDOPOTENTIAL STUDY OF (HGX2)2(X=F, CL, BR, I, H)

The preference of many solid mercury compounds for ''molecular'' struc tures with lower characteristic coordination numbers (frequently CN = 2) and lower boiling points than the corresponding zinc or cadmium spe cies is due to relativistic effects. In particular, the relativistic i ncrease of the mercury 6s-orbital ionization energies reduces the char ge separation in and the intermolecular interactions between HgX2 Mole cules containing electronegative substituents X. These are the major c onclusions of extensive quasirelativistic and nonrelativistic ab initi o pseudopotential Hartree-Fock and MP2 calculations on the dimeric sys tems (HgX2)2 (X = F, Cl, Br, I, H) and on the HgX2 monomers. While qua sirelativistic pseudopotential structure optimizations lead to weakly associated C2h complexes of two almost linear HgX2 units with Hg-X dis tances that are similar to those in the corresponding HgX2 solid-state structures, use of a nonrelativistic Hg pseudopotential results in sy mmetrically bridged D2h structures with far larger dimerization energi es. Only (HgH2)2 exhibits slightly unsymmetrical bridging even with th e nonrelativistic Hg pseudopotential. Natural population analyses (NPA ) and the electron localization function (ELF) have been employed to r ationalize the computed structural and thermochemical trends. While tr aditional explanations involving sd- or sp-hybridization arguments may have some bearing on the structures of HgH2 or of organomercury compo unds, electrostatic interactions and their relativistic reduction seem to be more important for the structural chemistry of mercury dihalide s and similar compounds with electronegative ligands.