Structure and stereochemistry in f-block complexes of high co-ordination number. Part 5. Ten-co-ordination : the crystal structures of tetrapotassium tetraoxalatouranate(IV) tetrahydrate(orthorhombic and triclinic phases), bicapped square antiprismatic and sphenocoronal stereochemistries

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Abstract
The title compond, K4[U(C2O4)4]·4H2O, has been prepared in two crystalline modifications; the crystal structures of both have been determined by single-crystal X-ray diffraction methods at 295 K. One form is orthorhombic, space group Fdd2, with unit-cell dimensions a= 30.09(2), b= 22.18(1), c= 12.747(6)Å, and Z= 16; R= 0.043 was obtained for 3 127 ‘observed’ reflections; the other form is triclinic, space group P, with unit-cell dimensions a= 9.595(5), b= 12.998(4), c= 10.329(5)Å, α= 115.47(3), β= 80.49(4), γ= 113.09(13)°, and Z= 2; R= 0.034 for 6 017 independent reflections. In each case, the formula unit is the asymmetric unit; some disorder occurs among the cations in the orthorhombic phase. In both structures the uranium atom is ten-co-ordinate, one of the ligands in each case being bridging and linking the uranium atoms in an infinite one-dimensional polymeric array. For the orthorhombic phase U–O range between 2.37(1) and 2.58(1)Å, while in the triclinic form the range is 2.406(5)–2.641(4)Å. The co-ordination stereochemistries about the uranium atoms are examined in terms of a repulsion model.

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