Second-Sphere Hydration of Rhodium(III) and Chromium(III) in Aqueous Solution. A Large-Angle X-Ray Scattering and EXAFS Study.

Abstract

The hydration structure around Rh(III) and Cr(III) ions in 1 M aqueous perchlorate solution has been studied by X-ray scattering measurements. Isostructural substitution was used to eliminate all contributions from atom-pair distances not involving the metal ions, leading to a radial distribution function dominated by the hydration of rhodium(III). The first hydration sphere contains six water molecules with the Rh-O distance 2.03 +/- 0.02 angstrom, and the well defined second hydration sphere holds 13 +/- 1 water molecules at a mean Rh-O distance of 4.02 +/- 0.02 angstrom. A hydrogen bond length between the first- and second-sphere water molecules of 2.63 +/- 0.09 angstrom was calculated using reasonable assumptions about the geometry of the coordinated water molecules in rhodium(III) and chromium(III) solutions. From EXAFS studies of Rh(ClO4)3.6H2O, as a solid and in aqueous solution, no significant structural differences in the first hydration sphere of the [Rh(H2O)6]3+ ion were found. No evidence for the existence of the well established second hydration sphere was found in our EXAFS data.