Gold(I) and mercury(II)-isoelectronic ions with strongly different chemistry: ab initio QMCF molecular dynamics simulations of their hydration structure.

The hydration structure of the isoelectronic Au(I) and Hg(II) ions was determined by means of ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulations. The two hydrates proved as very labile but entirely different in their structural features. While Hg(II) forms two distinct hydration shells, Au(I) is characterized by an additional extended first shell (meso-shell) which has a considerable influence on all data extracted from the simulation trajectory, namely, radial and angular distribution functions, coordination number distribution, and dynamical data such as mean ligand residence times (MRT) and vibrational frequencies. The short MRT values of the first shell ligands, amounting to a few picoseconds, lead to the simultaneous presence of a number of hydrate complexes with differing geometries, which explains the difficulties in assigning structural data to spectroscopic measurements. The results presented here demonstrate that isoelectronic transition metal ions can show strongly different chemical properties, which cannot be explained on the basis of their different charge alone. The importance of including the second hydration shell and thus the intershell hydrogen bonds in the quantum mechanical treatment of the simulation is clearly proven.