THE MERCURY-MERCURY BOND IN INORGANIC AND ORGANOMETALLIC COMPOUNDS - A THEORETICAL-STUDY

The effects of relativity and electron correlation in diatomic mercury HgX compounds and in linear HgX(2) and Hg(2)X(2) compounds (X=H, F, Cl, CH3 and CF3) are investigated using relativistic pseudopotential and local density approximation calculations. The stability of the Hg-Hg bond in Hg(2)X(2) is found to be influenced significantly by the electronegativity of the ligand X. Electronegative ligands such as fluorine (X=F) lead to strong mercury based radical character in the HgX monomer unit supporting an Hg-Hg bond to form a stable Hg(2)X(2). This effect diminishes for less electronegative ligands such as X=H or CH3 where the unpaired electron is localized more towards the ligand X. As a consequence, Hg-Hg bonding in Hg-2(CH3)(2) is very weak which may explain why organomercury compounds of the form Hg(2)R(2) (R=any organic group) have not yet been observed. Among these organomercury compounds Hg-2(CF3)(2) will be most stable. Relativistic and electron correlations effects are shown to have a significant influence on this trend. Hg-2(CR(3))(2) is isolobal to Au-2(PR(3))(2) and it is proposed that the Hg-2(CR(3))(2) unit may be stabilized at other metal centres.