The Chemistry of the trivalent actinoids. Part 7 [1]. Crystal structure analysis of [NH4]U[SO4]2·4H2O and comments on the structure of U2[SO4]3·9H2O

The crystal structure of an hydrated uranium(III) compound has been determined for the first time. Crystals of [NH4]U[So4]2·4H2O are monoclinic with a = 6.7065(2), b = 19.0328(6), c = 8.8305(3) A, β = 97.337(10°, z = 4 and space group P21/c. The structure was solved by the heavy-atom method from Cu-Kα diffractometer data, and refined by full-matrix least aquares to R = 0.096 for 1647 observed reflections. The compounds is isostructural with [NH4]M[SO4]2·4H2O (M = LaTb, except Pm). Each uranium atom is coordinated to nine oxygen atoms; six of them are contributed by four sulphate groups and the remaining three are from water molecules. The uranium-oxygen bond lengths were 2.37-2.60 A (to sulphate ion) and 2.47-2.56 A (to water). Each asymmetric unit contains a non-coordinated water molecule. The X-ray powder diffraction photograph of a substance previously reported as U2[So4]3·8H2O and that of La2[SO4]3·8H2O showed that compounds are isomorphous, with space group P63/m. Thus, the stoichiometry U2[So4]3·9H2O is more likely with nine/twelve-coordination found for the lanthanoid(III) sulphate octahydrates and Am2[SO4]3·8H2O. By comparing the diffuse reflectance electronic spectra of the title compounds with those of various uranium(II) solutions, it is confirmed that both water and sulphate ion have similar nephelazuxetic factors.