Structural Studies Coupling X-ray Diffraction and High-Energy X-ray Scattering in the UO₂²⁺–HBr_aq 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, Rb2UO2Br4·2H2O, K2UO2Br4·2H2O, and (NH4)2UO2Br4·2H2O, as well as a molecular dimer of uranium(VI), (UO2)2(OH)2Br2(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.