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 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.