Coordination of a uraniumIV sulfate monomer in an aqueous solution and in the solid state.

UraniumIV sulfate in an aqueous solution and the solid state has been investigated with extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD). The coordination polyhedron comprises monodentate sulfate, bidentate sulfate, and water molecules. The coordination modes of sulfate in solution have been determined from the U-S distances with EXAFS. The U-S distance of 3.67 +/- 0.02 A indicates monodentate sulfate, and the U-S distance of 3.08 +/- 0.02 A indicates bidentate coordination. The obtained sulfur coordination numbers of a solution with a [SO42-]/[U4+] ratio of 40 suggest species with compositions of [U(SO4,bid)2(SO4,mon)2 x nH2O]4- and [U(SO4,bid)3 (SO4,mon)2 x mH2O]6-. Charge-compensating countercations or ion pairing with Na+ and NH4+ could not be detected with EXAFS. One of the solution species, [U(SO4)5H2O]6-, has been conserved in a crystal. The corresponding crystal structure of Na1.5(NH4)4.5[U(SO4)5 x H2O] x H2O [space group P1, a = 9.4995(16) A, b = 9.8903(16) A, c = 12.744(2) A, alpha = 93.669(2) degrees, beta = 103.846(2) degrees, gamma = 109.339(2) degrees] has been determined by single-crystal XRD. Two monomeric uraniumIV sulfate complexes and three sodium units are linked in alternating rows and form a one-dimensional ribbon structure parallel to the a axis.