Coordination structure of Cu2+ ion in highly concentrated aqueous CuBr2 solution, determined by X-ray diffraction analysis.

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The solute structure of concentrated aqueous copper(II) bromide solutions was determined by the analysis of the radial distribution curves of X-ray diffraction measured on the solutions with varying concentrations, 4.364–1.016 mol dm−3. The mixed-ligand aquabromocopper(II) complexes were found to be present in these solutions, and to have a distorted octahedral geometry. The Cu2+ ion was coordinated with Br− ions at the equatorial sites whose bond lengths were shorter than the axial sites. The interatomic distances of the equatorial Cu–Br and Cu–OH2 bonds were, respectively, 2.42–2.46 and 1.96–1.99 Å, and that of the axial Cu–OH2 bond, 2.37–2.51 Å. The average coordination number of Br− ions in the nearest neighbor of the Cu2+ ion increased from 0.33 to 1.31 as the concentration of CuBr2 increased from 1.016 to 4.364 mol dm−3. Even in the 4.364 mol dm−3 solution, which was nearly saturated, the coordination of the Br− ion at the axial site was not evidenced. In the 1.474 mol dm−3 CuBr2 solution dissolved in an aqueous 6.923 mol dm−3 HBr solution, the tetrabromocopper(II) complex species was proposed to exist in a flattened tetrahedral form, wherein the Cu–Br distance was 2.41 Å.
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