Co-ordination chemistry of the methylmercury(II) ion in aqueous solution: a thermodynamic investigation

Reactions of the methylmercury(II) ion, CH3Hg+, have been investigated by potentiometry and calorimetry (0.15 mol dm−3 NaClO4 in water). The ligands used were hydroxide, chloride, bromide, iodide, acetate, methylamine, dimethylamine, 1,2-diaminoethane, 1,3-diaminopropane, 1,2,3-triaminopropane, tris(hydroxymethyl)aminomethane, glycinate, iminodiacetate, thioglycolate, (2-thioethanoate, SCH2CO2 2−), and cysteinate. The stability of the complexes is highly dependant on the type of donor atom involved in the formation of the co-ordinate bond. Complexes containing one or more Hg–S bonds are the most stable. With ligands that contain more than one donor site a set of complexes was found in which methylmercury(II) ions replace the protons of a fully protonated ligand, one by one. The standard enthalpy and entropy changes of formation of a complex with a polydentate ligand are compared to the values for the corresponding monodentate ligand. This comparison has provided strong evidence for the existence of chelated species in solution, perhaps in equilibrium with non-chelated species. The thermodynamic functions for the stepwise formation of complexes with ligands having various donor atoms have been used to propose which metal–ligand bond is the most favoured.