Exploring the hydration of Pb2+:: Ab initio studies and first-principles molecular dynamics

Even though lead is a well-known toxicant widely scattered throughout the world since antiquity, its chemistry is poorly documented at the molecular level. Here we investigate the hydration of the Pb2+ ion by means of first-principles molecular dynamics (Car-Parrinello molecular dynamics, CPMD). We found that the hydrated cation is heptacoordinated in a dynamically holodirected arrangement roughly corresponding to a fluxional distorted pentagonal bipyramid. The time-averaged Pb-O bond length is especially large and amounts to 2.70 angstrom with an associated root-mean-square deviation of 0.26 angstrom. This results from a dynamic exchange between short (< 2.6 angstrom), intermediate (2.6-3.0 angstrom) and long (> 3.0 angstrom) Pb-O bonds. The latter very long Pb-O distance implies that the determination of the coordination number n, from experimental work may not necessarily yield values directly comparable to the theoretical value of n(c)=7, since not all experimental techniques would recognize such a long distance as a bond to the metal cation. Pronounced disorders are evidenced in the second shell, characteristic of a chaotropic cation, and exchanges between the first and second shells cannot be excluded on a timescale of a few tens of picoseconds.