Hydration of Hg2+in Aqueous Solution Studied by Neutron Diffraction with Isotopic Substitution

The structural parameters of Hg2+ hydration were studied in 0.225 mol/L solutions of Hg2+ in DNO3/D2O by means of neutron diffraction with isotopic substitution of 199Hg for natHg. It was found that Hg2+ is hydrated by a first solvation shell of six water molecules. The observed Hg−O and Hg−H distances are equal to 2.48 ± 0.05 and 3.08 ± 0.05 Å, respectively. The angle φ between the plane of the water molecule and the cation−water oxygen axis is ∼ 35°. The solvation of Hg2+ therefore mimics very closely that of Ca2+ (the Ca−O and Ca−H distances are 2.40 and 3.03 Å, respectively) and helps to account for the extreme toxicity of mercury(II). We note also that the Hg−O distance obtained in the neutron diffraction experiment is larger by ∼0.1 Å than that obtained by X-ray diffraction. This difference is consistent with a shift of the oxygen electron density toward the mercury cation due to the covalency of the Hg−O interaction.