EXAFS investigation of inner- and outer-sphere chloroaquo complexes of Cr3+ in aqueous solutions

The structure of four solutions containing the isolated chromochloroaquo complexes, [CrCln(H2O)(6-n)]((3-n)+) (n = 1-3), have been investigated with the aid of EXAFS. Synthesis and isolation processes are described. In the inner-sphere complexes, Cr3+ ions are surrounded by 5H(2)O + 1Cl(-) (species [I], 1 m), 4H(2)O + 2Cl(-) (species [II], 1 and 0.005 m), and 3H(2)O + 3Cl(-) (species [III], 0.005 m), with Cr-O and Cr-Cr average distances of 2.00 and 2.30 Angstrom, respectively. In the four solutions a second coordination sphere is detected between 3.9 and 4.5 Angstrom. In solution containing species [I], this sphere is formed by water molecules (similar to 14) at 3.97 Angstrom whereas for solutions containing [II] and [III], several outer-sphere chloride anions are detected. For species [II] the second coordination sphere is formed by 3.3 (1 m) or 4.9 (0.005 m) chloride ligands, depending upon concentration of the complex, at around 4.3 Angstrom and by 8.0 (1 m) or 7.2 (0.005 m) water molecules at 3.9 a. For species [III], where [HCl] = 12 M, this sphere is formed exclusively by seven chloride ligands at 4.3 Angstrom.