On the structure of monohalomercury(II) complexes in dimethylsulfoxide, pyridine and N,N-dimetylthioformamide solution

Abstract

The structures of the monohalomercury(II) complexes, [HgX]+, in dimethylsulfoxide (dmso), pyridine and N,N-dimethylthioformamide (dmtf) solution have been studied by means of EXAFS. All complexes in the Hg2+-X− systems, X = Cl, Br and I, in dmso and pyridine have the unique property with wide range of predominance of all individual complexes. This means that all complexes can be individually characterized in solution. The previously reported complex formation thermodynamics in dmso and pyridine predicted a substantial desolvation at the formation of the first complex. This study shows that the dmso and pyridine solvated [HgX]+ complexes have one solvent molecule strongly bound “trans” to the halide ion and another two weakly bound solvent molecules in the remaining positions of linearly distorted tetrahedra. This shows that a substantial desolvation of the hexasolvated mercury(II) ions takes place at the formation of the [HgX]+ complexes. The strongly bound dmso molecule binds through its oxygen atom to mercury(II) in the [HgCl]+ and [HgBr]+ complexes, while in the [HgI]+ complex through its sulfur atom. The Hg–X bonds are slightly shorter in the [HgX]+ complexes than in corresponding [HgX2] complexes, and the short Hg–O and Hg–N bond distances are ca. 0.35 and 0.25 Å shorter than in the octahedral hexakis(dmso)- and -(pyridine)mercury(II) solvates, respectively. In dmtf solution, the dmtf solvated mercury(II) ion is four-coordinate in tetrahedral fashion. The structures of the dmtf solvated [HgX]+ complexes show the same basic tetrahedral configuration as the mercury(II) solvate. This shows that a simple substitution reaction takes place at the formation of the [HgX]+ complexes in dmtf.