Solvent effects on the stability of mixed ligand complexes

The effect of solvents on the equilibrium MX₄^2- + Y^- ⇋ MYX₃^2- + X^-, where M = Cd²⁺ or Hg²⁺ and X and Y = Cl^-, Br^-, I^-, or CN^-, has been studied by quantitative Raman spectrophotometry. This technique, employing LiClO₄ as internal reference, permits the direct determination of the decrease in concentration of the tetrahedral MX₄^2- due to the formation of MYX₃^2-. The fraction [1 - α(MX₄^2-)] is thus a direct measure of the solvent effect on the mixed complex stability. It is shown that bulk solvent properties are not relevant, and only solvent molecules in the immediate vicinity of the anionic complex have an effect on its stability. Selective solvation in this second coordination sphere is exhibited by several solvents, and water is not always the preferred solvent.