Stereochemistry of tervalent aqua ions: low-temperature neutron diffraction structures of CsFe(SO4)2·12H2O and CsFe(SeO4)2·12H2O

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Abstract
The structures of CsFe(SO4)2·12H2O and CsFe(SeO4)2·12H2O have been investigated by neutron diffraction at 15 K. Both alums crystallise in the cubic space group Pa3, Z= 4, with a= 12.354(6)Å for the sulphate and 12.593(10)Å for the selenate. The structures were refined using 802 [CsFe(SO4)2·12H2O] and 1 059 [CsFe(SeO4)2·12H2O] inequivalent reflections to give weighted R factors of 0.029 and 0.028 respectively. In agreement with prior X-ray crystallographic determinations, CsFe(SO4)2·12H2O gives the β alum modification whereas CsFe(SeO4)2·12H2O forms an α alum. The stereochemistry of water co-ordination to FeIII differs in the two alums, with the angle between the metal (III)–water bond and the plane of the co-ordinated water molecule being 0.6(10)° for the sulphate and 18.6(10)° for the selenate. This tilting of the plane of the co-ordinated water molecule is shown to be related to the alum type. The iron(III)–water bond distance shows some sensitivity to the tilt of the co-ordinated water molecule, lengthening from 1.994(1) to 2.002(1)Å as the plane of the water molecule is tilted by 18.6°. The sensitivity of the alum type to the size of the constituent ions, and to the stereochemistry of water co-ordination to the tervalent cation, is discussed in light of the hydrogen bonding in the alum structures.

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