Structural Studies Coupling X-ray Diffraction and High-Energy X-ray Scattering in the UO22+-HBraq System

The structural chemistry of uranium(VI) in concentrated aqueous hydrobromic acid solutions was investigated using both single crystal X-ray diffraction and synchrotron-based high-energy X-ray scattering (HEXS) to reveal the structure of the uranium(VI) complexes in solution prior to crystallization. The crystal structures of a series of uranyl tetrabromide salts are reported, including Cs2UO2Br4, Rb2UO2Br4 center dot 2H(2)O, K2UO2Br4 center dot 2H(2)O, and (NH4)(2)UO2Br4 center dot 2H(2)O, as well as a molecular dimer of uranium(VI), (UO2)(2)(OH)(2)Br-2(H2O)(4). Limited correspondence exists between the structures observed in the solid state and those in solution. Quantitative analysis of the HEXS data show an average U-Br coordination number of 1.9(2) in solution, in contrast to the U-Br coordination number of 4 in the solid salts.