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The structures of the UO2(aq)2+ ion and of the uranium(VI) hydroxide complex(es) formed in strongly alkaline solution have been investigated theoretically using molecular-orbital based quantum chemical methods, and experimentally using EXAFS methodology. Relativity was included explicitly through the Douglas−Kroll transformation. The uranium atom was described at the ECP level, using the AIMP methodology. The structures of [UO2(H2O)5]2+, and the hydroxide complexes, viz., [UO2(OH)4·(H2O)]2-, [UO2(OH)4]2-·(H2O), [UO2(O)(OH)2]2-·2(H2O), and [UO2(OH)5]3-, were optimized at the SCF level, using gradient techniques, while the relative stabilities were calculated at the MP2 level of approximation. The third structure contains three coordinated ligands, one of which is an oxide ion, in the plane perpendicular to the linear UO2-unit. Complexes of this type have not been experimentally identified for U(VI); however, they are formed for the iso-electronic Np(VII). The experimental EXAFS data indicates that the complex(es) formed is(are) mononuclear. The number of coordinated ligands in the equatorial plane is 4.5 ± 0.4, while the bond distances are the same within the experimental errors, as in a previous study of [Co(NH3)6 3+]2[UO2(OH)4 2-]3·2H2O, by Clark et al. An EXAFS model where the coordination number is fixed to four, is only marginally less precise than the model without constraints on the coordination number. This fact together with the close agreement between experimental and theoretically observed variations in bond distances between the different structure models provides a strong indication for the formation of [UO2(OH)4]2- in solution. This is an unusual coordination number for uranium(VI) complexes, previously found in sterically crowded systems such as UO2Cl4 2-.

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