A comparative UV-Vis and EXAFS investigation of the coordination of the uranyl ion UO22+ in solution

The identification of uranyl complexes in solution is not always straightforward. Unlike crystal structures, which can be revealed by X-ray diffraction, the structure of solution species could only be indicated in the past by means of spectroscopic techniques (UV-Vis absorption spectroscopy, luminescence, magnetic circular dichroism). These methods demonstrated the existence of a uranyl tetrachloro complex [UO2Cl4]2- with D4h coordination symmetry and a uranyl trinitrato complex [UO2(NO3)3]- with D3h symmetry in non-aqueous solution, based on their characteristic absorption spectra.Nowadays, limited structural parameters of solution species can be determined using Extended X-ray Absorption Fine Structure (EXAFS). The complex formation of the uranyl ion UO22+ with small inorganic ligands like Cl- and NO3- in non-aqueous solution was investigated using U LIII EXAFS spectroscopy and UV-Vis absorption spectroscopy. Both techniques confirm the existence of a [UO2Cl4]2- limiting species with D4h symmetry as well as a [UO2(NO3)3]- limiting species with D3h symmetry in acetonitrile solution. The distances in the U(VI) coordination sphere of [UO2Cl4]2- are U-Oax = 1.77 ± 0.01 Ǻ and U-Cl = 2.68 ± 0.01 Ǻ. For [UO2(NO3)3]- the distances in the U(VI) coordination sphere are U-Oax = 1.77 ± 0.01 Ǻ and U-Oeq = 2.49 ± 0.01 Ǻ. The U-N distance of 2.94 ± 0.01 Ǻ points at a bidentate coordination mode of the nitrate group.