Coordination of a uranium(IV) sulfate monomer in an aqueous solution and in the solid state

Uranium(IV) 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 angstrom indicates monodentate sulfate, and the U-S distance of 3.08 +/- 0.02 A indicates bidentate c oordination. The obtained sulfur coordination numbers of a solution with a SO4 (2-)]/[U4+] ratio of 40 suggest species with compositions of, [U(SO4,bid)(2)(SO4,(mon))(2)center dot nH(2)O](4-) and [U(SO4,bid)(3) (SO4,(mon))(2)center dot mH(2)O](6-). Charge-compensating countercations or ion pairing with Na+ and NH4+ could not be detected with EXAFS. One of the solution species, [U(SO4)(5)H2O!
(4.5)[U(SO4)(5 center dot)H2O]center dot H2O [space group P (1) over bar, a = 9.4995(16) angstrom, b = 9.8903(16) angstrom, c = 12.744(2) angstrom, alpha = 93,669(2)degrees, beta = 103.846(2)degrees,gamma = 109.339(2)degrees] has been determined by single-crystal XRD. Two monomeric uranium(IV) sulfate complexes and three sodium units are linked in alternating rows and form a one-dimensoinal ribbon structure parallel to the a axis.