Vibrational spectroscopic and force field studies of copper(II) chloride and bromide compounds, and crystal structure of KCuBr3

Stepakova, Liubov V. and Skripkin, Mikhail Yu and Chernykh, Lyudmila V. and Hajba, László and Mink, János (2008) Vibrational spectroscopic and force field studies of copper(II) chloride and bromide compounds, and crystal structure of KCuBr3. JOURNAL OF RAMAN SPECTROSCOPY, 39 (1). pp. 16-31. ISSN 0377-0486

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Abstract

Vibrational spectroscopic and force field studies have been performed of 15 related copper(II) chloride and copper(II) bromide compounds, including hydrated salts crystallizing in ternary aqueous systems with alkali and ammonium halides. For halocuprates with distorted octahedral coordination characteristic stretching Raman wavenumbers, corresponding to symmetric stretching Cu-II-X modes in the equatorial plane, were found in the ranges 247-288 cm(-1) for X = Cl, and 173-189 cm(-1) for X = Br, while the low-wavenumber stretching modes for the weaker axial Cu-X interactions varied considerably. The tetrahedral coordination for Cs2CuCl4 and Cs2CuBr4 leads to somewhat lower Cu-X symmetric stretching wavenumbers, 295 and 173 cm-1, respectively. The assignments of the copper-ligand stretching vibrations were performed with the aid of normal coordinate calculations. Correlations between force constants, averaged Cu-X stretching wavenumbers and bond distances have been evaluated considering the following aspects: (1) Jahn-Teller tetragonal distortion (axial elongation) of the octahedral copper(II) coordination environment, (2) differences between terminal and bridging halide ligands (3) effects of coordinated water and the influence of outer-sphere cations. Force constant ratios for terminal and bridging metal-halide bonds reveal characteristic differences between planar and tetrahedrally coordinated M2X6 species. In the hydrated copper(II) halide complexes, the halide ligands are more strongly bound than coordinated water molecules. The crystal structure of KCuBr3 (K2Cu2Br6), which was determined to provide structural information for the force field analyses, contains stacks of planar dimeric [Cu2Br6](2-) complexes held together by weak axial Cu-Br interactions.

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