An EXAFS spectroscopic, large-angle X-ray scattering, and crystallographic study of hexahydrated, dimethyl sulfoxide and pyridine 1-oxide hexasolvated mercury(II) ions

The structure of the solvated mercury(II) ion in water and dimethyl sulfoxide has been studied by means of large-angle X-ray scattering (LAXS) and extended X-ray absorption fine structure (EXAFS) techniques. The distribution of the Hg-O distances is unusually wide and asymmetric in both solvents. In aqueous solution, hexahydrated [Hg(OH2)(6)](2+) ions in a distorted octahedral configuration, with the centroid of the Hg-O distance at 2.38(1) angstrom, are surrounded by a diffuse second hydration sphere with Hg center dot center dot center dot O-II,distances of 4.20(2) angstrom. In dimethyl sulfoxide, the six Hg-O and Hg center dot center dot center dot S distances of the hexasolvated [Hg{OS(CH3)(2)}(6)](2+) complex are centered around 2.38(1) and 3.45(2) angstrom, respectively. The crystal structure of hexakis(pyridine 1-oxide)mercury(II) perchlorate has been redetermined. The space group R! implies six equal Hg-O distances of 2.3416(7) angstrom for the [Hg(ONC5H5)(6)](2+) complex at 100 K. However, EXAFS studies of this compound, and of the solids hexaaquamercury(II) perchlorate and hexakis(dimethyl sulfoxide)mercury(II) trifluoromethanesulfonate, also with six equidistant Hg-O bonds according to crystallographic results, reveal in all cases strongly asymmetric Hg-O distance distributions. Vibronic coupling of valence states in a so-called pseudo-Jahn-Teller effect probably induces the distorted configurations.