Low-temperature neutron-diffraction structure of [Ru(OH2)6]3+in the caesium sulphate alum lattice CsRu[SO4]2·12H2O

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Abstract
The salt CsRu[SO4]2·12H2O has been investigated by single-crystal neutron diffraction at 15 K. The crystals belong to the cubic space group Pa, Z= 4, with a= 12.371 (6)Å. The structure, which conforms to the β alum modification, was refined using 894 inequivalent reflections to give a weighted R factor of 0.026. The stereochemistry of the water co-ordination to RuIII is trigonal planar, with the angle between the metal (III)–water bond and the plane of the co-ordinated water molecule being 0.8(6)°. The angle between the plane of the co-ordinated water molecule and the RuO6 octahedron is 22.0(6)°, midway between the positions appropriate for Th and all-horizontal D3d symmetry of [Ru(OH2)6]3+. The anisotropic thermal ellipsoids of the oxygen atom co-ordinated to RuIII are large and of a shape similar to that of its co-ordinated hydrogen atoms. This observation is ascribed to a softening of the potential about the water co-ordinated to RuIII. Single-crystal Raman spectra of CsRu[SO4]2·12H2O exhibit features which are assigned to soft modes and which indicate that the potential softens on cooling below ca. 90 K. In contrast, neither the low-temperature structure nor the single-crystal Raman spectra of the isomorph CsFe[SO4]2·12H2O exhibits features consistent with softening of the potential about the water co-ordinated to MIII. Since the sizes of the hexa-aqua cations of RuIII and FeIII are similar it is concluded that the electronic structure of RuIII is implicated in the softening of the potential about its co-ordinated water molecule. The trigonal-field splitting of the 2T2g(Oh) ground term is discussed in terms of the overall stereochemistry of [Ru(OH2)6]3+.