EXAFS Investigation of Inner- and Outer-Sphere Chloroaquo Complexes of Cr3+ in Aqueous Solutions

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Abstract

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 5H2O + 1Cl- (species [I], 1 m), 4H2O + 2Cl- (species [II], 1 and 0.005 m), and 3H2O + 3Cl- (species [III], 0.005 m), with Cr−O and Cr−Cl average distances of 2.00 and 2.30 Å, respectively. In the four solutions a second coordination sphere is detected between 3.9 and 4.5 Å. In solution containing species [I], this sphere is formed by water molecules (∼14) at 3.97 Å, 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 4.4 Å and by 8.0 (1 m) or 7.2 (0.005 m) water molecules at 3.9 Å. For species [III], where [HCl] = 12 M, this sphere is formed exclusively by seven chloride ligands at 4.3 Å.

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