Relationship between the electronic and molecular structure of tervalent aqua ions: low-temperature neutron diffraction structure of CsCr(SO4)2·12H2O

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Abstract
The alum CsCr(SO4)2·12H2O has been investigated by single-crystal neutron diffraction at 15 K. The structure, which conforms to the β modification, was refined using a total of 651 unique reflections to give a weighted R value of 0.041. The stereochemistry of water co-ordination to chromium(III) is trigonal planar, with the Cr–O bond vector making an angle of 0.8(6)° with the plane of the co-ordinated water molecule. The angle between the plane of the co-ordinated water molecule and the CrO6 framework, φ, is –19.0(4)°, similar to that found for the corresponding iron salt [–19.4(3)°], but smaller than found for the corresponding ruthenium [–22.0(3)°] and vanadium [–22(1)°] salts. The relationship between φ and the splitting of the t2g(Oh) orbital energies is described using the angular overlap model. Accordingly, the preferred value of φ depends on the occupancy of the t2g orbitals. Using this approach the φ values found for the caesium sulphate β alums are explained, and the electronic structures of the titanium and vanadium hexaaqua cations are discussed. The general features of the site of the tervalent cation in the alum lattice are described, these being important owing to the extensive use of the alum lattice for the spectroscopic study of tervalent aqua ions.

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