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-.

The structures of the complexes UO(2)F(n)(H(2)O)(5-n)(2-n), n = 3-5, have been studied by EXAFS. All have pentagonal bipyramid geometry with U-F of and U-H(2)O distances equal to 2.26 and 2.48 A, respectively. On the other hand the complex UO(2)(OH)(4)(2-) has a square bipyramid geometry both in the solid state and in solution. The structures of hydroxide and fluoride complexes have also been investigated with wave function based and DFT methods in order to explore the possible reasons for the observed structural differences. These studies include models that describe the solvent by using a discrete second coordination sphere, a model with a spherical, or shape-adapted cavity in a conductor-like polarizable continuum medium (CPCM), or a combination of the two. Solvent effects were shown to give the main contribution to the observed structure variations between the uranium(VI) tetrahydroxide and the tetrafluoride complexes. Without a solvent model both UO(2)(OH)(4)(H(2)O)(2-) and UO(2)F(4)(H(2)O)(2-) have the same square bipyramid geometry, with the water molecule located at a distance of more than 4 A from uranium and with a charge distribution that is very near identical in the two complexes. Of the models tested, only the CPCM ones are able to describe the experimentally observed square and pentagonal bipyramid geometry in the tetrahydroxide and tetrafluoride complexes. The geometry and the relative energy of different isomers of UO(2)F(3)(H(2)O)(2-) are very similar, indicating that they are present in comparable amounts in solution. All calculated bond distances are in good agreement with the experimental observations, provided that a proper model of the solvent is used.

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The local structure of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution was investigated by U-L(3) and Th-L(3) EXAFS spectroscopy for total sulfate concentrations 0.05 < or = [SO(4)(2-)] < or = 3 M and 1.0 < or = pH < or = 2.6. The sulfate coordination was derived from U-S and Th-S distances and coordination numbers. The spectroscopic results were combined with thermodynamic speciation and density functional theory (DFT) calculations. In equimolar [SO(4)(2-)]/[UO(2)(2+)] solution, a U-S distance of 3.57 +/- 0.02 Angstrom suggests monodentate coordination, in line with UO(2)SO(4)(aq) as the dominant species. With increasing [SO(4)(2-)]/[UO(2)(2+)] ratio, an additional U-S distance of 3.11 +/- 0.02 Angstrom appears, suggesting bidentate coordination in line with the predominance of the UO(2)(SO(4))(2)(2-) species. The sulfate coordination of Th(IV) and U(IV) was investigated at [SO(4)(2-)]/[M(IV)] ratios > or = 8. The Th(IV) sulfato complex comprises both, monodentate and bidentate coordination, with Th-S distances of 3.81 +/- 0.02 and 3.14 +/- 0.02 Angstrom, respectively. A similar coordination is obtained for U(IV) sulfato complexes at pH 1 with monodentate and bidentate U-S distances of 3.67 +/- 0.02 and 3.08 +/- 0.02 Angstrom, respectively. By increasing the pH value to 2, a U(IV) sulfate precipitates. This precipitate shows only a U-S distance of 3.67 +/- 0.02 Angstrom in line with a monodentate linkage between U(IV) and sulfate. Previous controversially discussed observations of either monodentate or bidentate sulfate coordination in aqueous solutions can now be explained by differences of the [SO(4)(2-)]/[M] ratio. At low [SO(4)(2-)]/[M] ratios, the monodentate coordination prevails, and bidentate coordination becomes important only at higher ratios.

The structures of aqueous M(4+)(aq) and MF(3+)(aq), where M is uranium(IV) or thorium(IV), have been determined by L(III) edge EXAFS using data from solutions of 1.5 M HClO(4) in which the M(IV) concentrations ranged from 0.03 to 0.3 M. A least-squares refinement of the data for the aqua ions indicated 10.8 +/- 0.5 water molecules in the first hydration sphere of both ions and M-O bond distances for U(IV) and Th(IV) of 2.42 +/- 0.01 and 2.45 +/- 0.01 Å, respectively. By considering both previous structure information and the EXAFS data, we selected N = 10 +/- 1 as the most likely coordination number of both M(IV) aqua ions. EXAFS measurements from acidic aqueous uranium(IV) and thorium(IV) solutions containing fluoride show that large changes in the first coordination sphere occur. The experimental data indicates an asymmetrical distribution of the distances, probably as a result of differing M-F and M-O bond lengths. These can be described by a model that contains two different bond distances, one M-F distance at 2.10 Å and one M-O distance at 2.45 Å for U(IV); for Th(IV), the corresponding distances are 2.14 and 2.48 Å. The total coordination number in this model is unchanged from the aqua ions, i.e., 10 +/- 1.

The structure of dioxouranium(VI) as a function of pH at different (CHIn strong alkaline solutions [>1 M (CH

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The bond distances and coordination numbers of the predominant Np(VII) complex in strongly alkaline solution have been determined using EXAFS transmission measurements. The stoichiometry and structure of NpO 4 (OH) 2 3has been deduced by combining these data with different structure models, mostly determined by using DFT based methods. The experimental and theory based distance Np(VII)-O oxo is 1.89 4 and 1.90 Å, respectively, whereas the Np(VII)-OHdistance is 2.32 6 and 2.33 Å, respectively. Theory based geometry and bond distances have been obtained also for other Np(VII) and Np(VI) complexes, NpO 2 (OH) 4 -, NpO 4 -, NpO 4 (OH) 2 4-, and NpO 2 (OH) 4 2-, NpO 4 2-. The "NpO 6 "unit has a square bipyramidal geometry both in NpO 4 (OH) 2 3-/4and in NpO 2 (OH) 4 1-/2-, albeit with some difference in bond distances. The close similarity in structure indicates that no major rearrangements are necessary on electron transfer between Np(VI) and Np(VII), a possible explanation for the stable and reproducible Np(VII)/Np(VI) redox potential observed in alkaline solution. The structure data indicate that new Np(VII) species may be identified by oxidation of Np(VI) solutions at lower hydroxide concentrations.