Molecular view of the anomalous acidities of Sn2+, Pb2+ and Hg2+

Experimental results taken from both the condensed and gaseous phase show that, when associated with water, the three dications Sn2+, Pb2+, and Hg2+ exhibit a facile proton-transfer reaction. In the gas phase, no stable [M.(H2O)(n)](2+) ions are observed; but instead the cations appear to undergo rapid hydrolysis to give ions of the form M+OH(H2O)(n-1). A series of ab initio calculations have been undertaken on the structures and proton-transfer reaction profiles associated with the complexes [M.(H2O)2,4](2+), where M is one of Sn, Pb, Hg, and Ca. The latter has been used as a reference point both in terms of comparisons with previous calculations, and the fact that Ca2+ is a very weak acid. The calculations show that for Sn2+, Pb2+, and Hg2+, the only barriers to proton transfer are those associated with the movement of water molecules. In the gas phase, these barriers could be overcome through energy gained during ion formation, and in the condensed phase the thermal motion of water molecules would be sufficient. In contrast, the calculations show that for Ca2+ it is the proton-transfer step that provides the most significant reaction barrier. Proton transfer in Sn2+ and Pb2+ is further assisted by distortions in the geometries of [M.(H2O)2,4](2+) complexes due to voids created by the 5S(2) (6S(2)) inert lone pair. For Hg2+, ease of proton transfer is derived partly from the high degree of covalent bonding found in both the reactants and products.