Solubilization and coordination of the HgCl2 molecule in water, methanol, acetone, and acetonitrile: an X-ray absorption investigation

Abstract

X-ray absorption spectroscopy (XAS) has been employed to carry out structural characterization of the local environment around mercury after the dissolution of the HgCl₂ molecule. A combined EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) data analysis has been performed on the Hg L₃-edge absorption spectra recorded on 0.1 M HgCl₂ solutions in water, methanol (MeOH), acetone and acetonitrile. The Hg–Cl distance determined by EXAFS (2.29(2)–2.31(2) Å) is always comparable to that found in the HgCl₂ crystal (2.31(2) Å), demonstrating that the HgCl₂ molecule dissolves in these solvents without dissociating. A small sensitivity of EXAFS to the solvent molecules interacting with HgCl₂ has been detected and indicates a high degree of configurational disorder associated with this contribution. XANES data analysis, which is less affected by the disorder, was therefore carried out for the first time on these systems to shed light into the still elusive structural arrangement of the solvent molecules around HgCl₂. The obtained results show that, in aqueous and MeOH solutions, the XANES data are compatible with three solvent molecules arranged around the HgCl₂ unit to form a trigonal bipyramidal structure. The determination of the three-body Cl–Hg–Cl distribution shows a certain degree of uncertainty around the average 180° bond angle value, suggesting that the HgCl₂ molecule probably vibrates in the solution around a linear configuration.