High-level ab initio calculation of the stability of mercury-thiolate complexes

The reliability of ab initio methods to predict accurate thermodynamic properties and coordination geometries of mercury-thiolate complexes was examined with calculations at various levels of theory. The second-order Moller-Plesset perturbation theory (MP2) method in connection with the Stuttgart-Dresden-Bonn relativistic effective core potentials and the related correlation consistent valence basis set gives optimized Hg(RS) (n) model structures in good agreement with experimental data. Differences in thermodynamic stability among various models can be estimated with chemical precision using single-point energy calculation at the CCSD(T) level of theory performed on the MP2-optimized structures. This computational scheme was applied next to calculate the stability of aqueous linear (two coordinated), trigonal, and tetrahedral mercury-thiolate complexes. In alkaline solutions, the difference in complexation Gibbs free energy between the most stable (trigonal) and the less stable (tetrahedral) model complexes formed with free ligands is only -4.7 kcal mol(-1). At neutral pH, the linear coordination is most stable. When the thiol ligands are structurally associated, as in biological systems, the trigonal coordination is most stable from pH 4.8 to 10.6. The relative stabilities of the three Hg-(RS) (n) bonding configurations reported herein can be further modified in biological environment by Hg-induced folding of proteins.