Uranyl(VI) compounds. 3. Crystal structures of two forms of bis(urea)dioxouranium(VI) sulfate

The structures of the α and β forms of the title compound UO2SO4.2CO(NH2)2 are similar. The uranium atom lies at the center of a pentagonal bipyramid formed by the oxygen atoms. Of the 5 equatorial oxygens, 3 belong to sulfate groups and two to urea molecules. The bridging tridentate sulfate groups join the bipyramids into infinite double chains in the same fashion as in uranyl sulfate hydrates. Selected interatomic distances and angles are listed. The distances and angles in UO7 polyhedra are in close agreement with values reported for other related uranyl sulfates. The U-O (uranyl) bond distances in the α form are unusually long, exceeding 1.8 A. The U-O (urea) distances in the β are slightly longer than in the α form. The orientation of the planar area molecules is different in the 2 forms. The urea molecules are involved in the hydrogen bonding within and between the chains. The main structural difference between the 2 forms lies in the packing of the chains. The packing efficiency is slightly greater in the α form as the unit cell volume is 2.3% smaller. 2 figures, 5 tables