Evidence for Sevenfold Coordination in the First Solvation Shell of Hg(II) Aqua Ion

Abstract

A quite unexpected sevenfold coordination of the hydrated Hg(II) complex in aqueous solution is revealed by an extensive study combining X-ray absorption spectroscopy (XAS) and quantum mechanics/molecular dynamics (QM/MD) calculations. As a matter of fact, the generally accepted octahedral solvation of Hg(II) ion cannot be reconciled with XAS results. Next, refined QM computations point out the remarkable stability of a heptacoordinated structure with C2 symmetry, and long-time MD simulations by new interaction potentials including many-body effects reveal that the hydrated complex has a quite flexible structure, corresponding for most of the time to heptacoordinated species. This picture is fully consistent with X-ray absorption near-edge structure experimental data which unambiguously show the preference for a sevenfold instead of a sixfold coordination.

An EXAFS Spectroscopic, Large‐Angle X‐Ray Scattering, and Crystallographic Study of Hexahydrated, Dimethyl Sulfoxide and Pyridine 1‐Oxide Hexasolvated Mercury(II) Ions

“…An appreciably longer mean HgÀO distance would be expected for a seven-coordinated mercury(II) ion, as proposed by Chillemi et al, [22] compared with that observed for hexasolvated mercury(II) ions in the solid state. In a comparable case, for cadmium(II), the mean Cd À O distance in hexasolvated cadmium complexes in the solid state is significantly shorter than those observed in water and dimethyl sulfoxide, which supports the partial (ca.…”

“…[32] This observation is in contrast with the high amplitude of the EXAFS oscillation at high k values reported by Chillemi et al [22] (cf. Figure 1), which is probably an artifact caused by the very large and unrealistic C 3 value reported for the model refinement using the program package GNXAS, and/or the presence of a substantial amount of the hydrolysis complex [Hg(OH) 2 ] with short linear OÀHgÀO bonds.…”

“…[21] In a recent study that combined X-ray absorption spectroscopic (extended X-ray absorption fine structure (EXAFS) and extended adsorption near edge structure (XANES)) and QM/MD calculations using two-body pair potentials, a flexible seven-coordinated structure with C 2 symmetry was advocated instead of the generally accepted six-coordination. [22] The EXAFS analysis could not distinguish between six-, seven-, or eight-coordination and resulted in an extremely asymmetric distribution of HgÀO distances with a peak maximum at 2.32 , independent of the coordination number (CN), and with very large values for the width (s 2 ) and asymmetry (C 3 ) of the bond-distance distribution, 0.025 2 and 0.0065 3 , respectively, for CN = 6 (even somewhat larger for CN = 7 and 8). For six-coordination, our evaluation shows that the reported parameters would correspond to an Hg À O centroid value of 2.46 , assuming that the reported HgÀO distance is the peak maximum of the distribution.…”

Uptake of Toxic Metal Ions from Water by Naked and Monolayer Protected Silver Nanoparticles: An X-ray Photoelectron Spectroscopic Investigation

“…The cell emf (electromotive force) for the reaction, nAg(0) + M n+ f nAg + + M(0) (1) or xAg(0) + M n+ f xAg + + M (n-x)+ (2), is positive only for reaction 2 with Hg(II). But, this emf is not largely positive, so the reduction is not highly favored.…”

Hg(II) coordination complexes containing the tris(2-pyridylmethyl)amine ligand: Synthesis, characterization and crystal structure analysis

“…Recently, efforts have been made to better understand the coordination properties of this metal ion. In the course of these investigations, a quite unexpected sevenfold coordination of a hydrated Hg(II) complex in aqueous solution has been revealed [6]. Also, there are some examples reported in the literature describing the influence of weak intermolecular interactions on the coordination geometry of the metal center and the formation of unusual coordination geometries around the Hg(II) ion [7][8][9].…”