INNER-SPHERE AND OUTER-SPHERE COMPLEX-FORMATION IN AQUEOUS ERBIUM HALIDE AND PERCHLORATE SOLUTIONS - AN X-RAY-DIFFRACTION STUDY USING ISOSTRUCTURAL SUBSTITUTION

Intensity difference functions obtained from large angle X-ray diffraction measurements on isostructural erbium and yttrium halide and perchlorate solutions have been used to derive the coordination around the erbium ion in solutions of different concentrations and anion–metal ratios. Two well-defined coordination spheres can be distinguished in the radial distribution functions. In the first sphere the erbium ion coordinates 8.0 ± 0.3 H2O molecules at a distance of 2.35 ± 0.01 Å. Halide ions and possibly perchlorate ions penetrate into this sphere only when present in very large concentrations. In the inner-sphere complexes then formed, the Er–Cl and the Er–Br bonding distances are found to be 2.7 ± 0.1 and 2.87 ± 0.02 Å, respectively. The halide ions are concentrated to the second coordination sphere, in which the ratio between the numbers of halide ions and water molecules is larger than the stoichiometric ratio. In these solvent-separated ion pairs (outer-sphere complexes), the Er–Cl and Er–Br distances are found to be 5.0 Å. © 1990, American Chemical Society. All rights reserved.