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 HgX2 and Hg2X2 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 Hg2X2 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 Hg2X2. 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 Hg2(CH3)2 is very weak which may explain why organomercury compounds of the form Hg2R2 (R=any organic group) have not yet been observed. Among these organomercury compounds Hg2(CF3)2 will be most stable. Relativistic and electron correlations effects are shown to have a significant influence on this trend. Hg2(CR3)2 is isolobal to Au2(PR3)2 and it is proposed that the Hg2(CR3)2 unit may be stabilized at other metal centres.