Structures and energetics of erbium chloride complexes in aqueous solution

A series of aqueous, constant ionic strength solutions containing 0.5 m Er is studied as a function of chloride/perchlorate ion concentration using high-energy X-ray scattering (HEXS) to probe the metal ion coordination environment. Perchlorate is seen to form an outer-sphere complex only in the end member (4.0 m perchlorate) of the series. Chloride ions are seen to bind as both inner- and outer-sphere complexes. A quantitative analysis of the scattering data is used to determine stability constants that depend on whether the complexation with chloride ions is assumed to add to or to replace the waters bound in the first coordination sphere. Published stability constants obtained from liquid-liquid extraction clarify the mechanism involved and, together with the HEXS data, present a consistent picture of the chemistry. The stability constants determined from the HEXS data, beta(1) = 0.38(8) and beta(2) = 0.014(9), are small, less than the available thermal energy in solution at room temperature. The combination of chemical separations and HEXS data for the analysis supports the argument that the complexes formed are real under the experimental conditions. Details of the structural correlations and interactions are discussed for applications including separations and environmental modeling.