ORIGIN OF THE UNIQUE STABILITY OF CONDENSED-PHASE HG22- AN AB-INITIO INVESTIGATION OF MI AND MII SPECIES (M=ZN, CD, HG)( )

The stability of the Hg2(2+) cation and related species is due to diff erential aggregation/solvation effects in the condensed phase. These a re strongly modified by relativistic effects. Thus, relativity is resp onsible for the existence of Hg-Hg-bonded species, but only in the con densed phase, and the stability is not due to the relativistic strengt hening of the metal-metal bond itself, as suggested earlier. Ab initio pseudopotential calculations at theoretical levels higher than previo usly reported show that relativistic effects clearly shift the equilib rium Hg2X2(g) reversible HgX2(g) + Hg(g) (X = F, Cl, H) to the right a nd not to the left. There is a considerably greater chance to find the corresponding Zn-Zn- or Cd-Cd-bonded species in the gas phase! In the condensed phase, differential aggregation or solvation effects favor the Hg(n)2+ cations: (a) The shift of the equilibrium to the right by the aggregation energy of the elemental metal is less pronounced for M = Hg than for M = Zn and Cd, very likely due to relativity. (b) The r elativistic reduction of aggregation or solvation energies is larger f or HgX2 species than for the corresponding Hg2X2 compounds. This is sh own by calculations on molecular model systems, MCl2.H2O, M2Cl2.H2O, ( MF2)2, and (M2F2)2, and by periodic Hartree-Fock calculations on solid Hg2F2 and HgF2.