Hg-Hg bonding in mercurous Hg(I)(2)L(2) compounds: The influence of ligand electronegativity

Abstract

The influence of ligand electronegativity (EN) on the Hg-Hg bonding in mercurous compounds has been investigated by calculations on a series of Hg(2)L(2) species. The various calculated molecular properties are strongly dependent on the electronegativity of the attached ligand, where the EN of the ''central'' atom in the ligand (if the ligand L is a group) plays a major role. However, no strict correlation has been found. The EN value of 2.50 (value based on Mulliken's method) for the ''central'' atom appears to be a critical one for a classification of the compounds. For EN > 2.50, the variation of such molecular properties as Hg-Hg bond length, Hg-Hg force constant and reaction energy (Hg(2)L(2)-->HgL(2) + Hg) is small with different ligands L, and these compounds are all stable against disproportionation, whereas an abrupt change in these molecular properties (mainly the Hg-Hg bond length and the reaction energy) will occur on substitution by a ligand with ''central'' atom EN < 2.50, and also such compounds become rather unstable. This accounts for the fact that the Hg atoms are mostly bonded to halogen and oxygen in mercurous compounds. Also, the calculation supports the small variation of the Hg-Hg bond lengths for different halogens, as found by a recent X-ray single crystal determination. We have also discussed the effect of the CH3/CF3 substitution on the Hg-C bond length and other molecular properties. The calculated trends in most cases are opposite to the trends predicted from group electronegativities given in the literature, but are qualitatively consistent with some experimental evidence.