Coordination of a Uranium(IV) Sulfate Monomer in an Aqueous Solution and in the Solid State Christoph Hennig,* ,† Werner Kraus, ‡ Franziska Emmerling, ‡ Atsushi Ikeda, † and Andreas C. Scheinost † Forschungszentrum Dresden-Rossendorf, Institute of Radiochemistry, P.O. Box 510119, D-01314 Dresden, Germany, and Bundesanstalt für Materialforschung and -prüfung Berlin, Richard-Willstätter-Strasse 11, D-12489 Berlin, Germany Received September 21, 2007 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 Å indicates monodentate sulfate, and the U-S distance of 3.08 ( 0.02 Å indicates bidentate coordination. The obtained sulfur coordination numbers of a solution with a [SO 4 2- ]/[U 4+ ] ratio of 40 suggest species with compositions of [U(SO 4,bid ) 2 (SO 4,mon ) 2 ·nH 2 O] 4- and [U(SO 4,bid ) 3 (SO 4,mon ) 2 ·mH 2 O] 6- . Charge-compensating countercations or ion pairing with Na + and NH 4 + could not be detected with EXAFS. One of the solution species, [U(SO 4 ) 5 H 2 O] 6- , has been conserved in a crystal. The corresponding crystal structure of Na 1.5 (NH 4 ) 4.5 [U(SO 4 ) 5 ·H 2 O]·H 2 O [space group P1 ¯ , a ) 9.4995(16) Å, b ) 9.8903(16) Å, c ) 12.744(2) Å, R) 93.669(2)°,  ) 103.846(2)°, γ ) 109.339(2)°] 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. Introduction Coordination of hexavalent uranium sulfate in an aqueous solution has been extensively investigated by different spectroscopic techniques. 1–5 Because of small differences in the Gibbs free energy, the sulfate group is able to coordinate in monodentate as well as in bidentate fashion. 1,6 The solution species UO 2 SO 4 0 shows monodentate sulfate coor- dination, but this species also comprises to a small extent a bidentate-coordinated sulfate isomer. 1,7 The species UO 2 - (SO 4 ) 2 2- shows predominantly bidentate sulfate coordina- tion. 1 In contrast, studies on the coordination of tetravalent uranium sulfate in an aqueous solution are scarce. The current NEA thermodynamic database reports only two sulfate species, USO 4 2+ and U(SO 4 ) 2 (aq). 8 Uranium(IV) sulfate species occur as monomers with monodentate and bidentate sulfate coordination. 1 The fraction of bidentate coordination increases with an increasing [SO 4 2- ]/[U 4+ ] ratio. However, uranium(IV) often forms polynuclear complexes and poly- mers. 8,9 Known crystal structures of uranium(IV) sulfate show only ternary polymers with monodentate bridging sulfate ligands. 10–12 These structures result from a rearrange- ment of the sulfate group during the crystallization process and do not represent the solution species. The aim of this * To whom correspondence should be addressed. E-mail: hennig@esrf.fr. † Forschungszentrum Dresden-Rossendorf, Institute of Radiochemistry. ‡ Bundesanstalt für Materialforschung and -prüfung Berlin. (1) Hennig, C.; Schmeide, K.; Brendler, V.; Moll, H.; Tsushima, S.; Scheinost, A. C. Inorg. Chem. 2007, 46, 5882. (2) Moll, H.; Reich, T.; Hennig, C.; Rossberg, A.; Szabo, Z.; Grenthe, I. Radiochim. Acta 2000, 88, 559. (3) Neuefeind, J.; Skanthakumar, S.; Soderholm, L. Inorg. Chem. 2004, 43, 2422. (4) Gál, M.; Goggin, P. L.; Mink, J. Spectrochim. Acta A 1992, 48, 121. (5) Nguyen-Trung, C.; Begun, G. M.; Palmer, D. A. Inorg. Chem. 1992, 31, 5280. (6) Vallet, V.; Grenthe, I. C. R. Chim. 2007, doi:10.1016/j.crci. 2007. 03.004. (7) Hennig, C.; Ikeda, A.; Schmeide, K.; Brendler, V.; Moll, H.; Tsushima, S.; Scheinost, A. C.; Skanthakumar, S.; Wilson, R.; Soderholm, L.; Servaes, K.; Görrler-Walrand, C.; Van Deun, R. Radiochim. Acta, submitted. (8) Guillaumont, R.; Fanghänel, T.; Fuger, J.; Grenthe, I.; Neck, V.; Palmer, D. A.; Rand, M. H. Update on the chemical thermodynamics of uranium, neptunium, plutonium, americium and technetium; Elsevier Science Publishers: Amsterdam, The Netherlands, 2003. (9) Natrajan, L.; Mazzanti, M.; Bezombes, J.-P.; Pécaut, J. Inorg. Chem. 2005, 44, 6115. Inorg. Chem. 2008, 47, 1634-1638 1634 Inorganic Chemistry, Vol. 47, No. 5, 2008 10.1021/ic701880h CCC: $40.75  2008 American Chemical Society Published on Web 02/02/2008