The inner-sphere mechanisms of the disproportionation reactions of U(V), Np(V), and Pu(V) ions have been studied using a quantum mechanical approach. The U(V) disproportionation proceeds via the formation of a dimer (a cation−cation complex) followed by two successive protonations at the axial oxygens of the donor uranyl ion. Bond lengths and spin multiplicities indicate that electron transfer occurs after the first protonation. A solvent water molecule then breaks the complex into solvated U(OH)22+ and UO22+ ions. Pu(V) behaves similarly, but Np(V) appears not to follow this path. The observations from quantum modeling are consistent with existing experimental data on actinyl(V) disproportionation reactions.

A theoretical study of the inner-sphere disproportionation reaction mechanism of the pentavalent actinyl ions.

The chemistry of the uranyl ion ([UO(2)](2+)) has evolved remarkably over the past few years, with unexpected reactivity observed that challenge our understanding of this ion, and of actinides in general. This review highlights some recent advances in the field, focussing on the organometallic chemistry of the uranyl moiety, which is not well developed in comparison to lower oxidation states of uranium. The use of uranyl as a catalyst is highlighted and the newly developed supramolecular chemistry is described. The uranyl oxygen atoms have been considered as inert, but recent work has shown that is not necessarily the case and is discussed herein. Finally, reduction to the [UO(2)](+) ion will be discussed.

New Reactivity of the Uranyl(VI) Ion

The hydrothermal reaction of NpO(2) with IO(3)(-) in the presence of nitrate results in the formation of NpO(2)(IO(3)) (1). Under similar conditions, NpO(2) reacts with AgNO(3) and SeO(2) to yield alpha-AgNpO(2)(SeO(3)) (2) and beta-AgNpO(2)(SeO(3)) (3). The structure of 1 consists of distorted pentagonal bipyramidal Np(V) centers that are bridged by iodate anions. In addition, the oxo atoms of the neptunyl(V) cations coordinate adjacent Np(V) centers creating layers that are linked into a three-dimensional network structure by the iodate anions. The structure is polar owing to the alignment of the stereochemically active lone pair of electrons on the iodate anions along the c-axis. alpha-AgNpO(2)(SeO(3)) (2) forms a layered structure consisting of hexagonal bipyramidal NpO(8) polyhedra that are bound by chelating and bridging selenite anions. The primary and secondary structures of 3 are similar to those of 1, and neptunyl-neptunyl interactions are partially responsible for the creation of a three-dimensional network structure. However, the selenite anions in 3 are rotated with respect to the iodate anions found in 1, and the structure is centrosymmetric. The network found in 3 consists of interconnecting, approximately square channels that house the Ag(+) cations. A bond-valance sum parameter of 2.036 A for Np(V) bound exclusively to oxygen has been developed with b = 0.37 A. Crystallographic data: 1, orthorhombic, space group Pna2(1), a = 13.816(2) A, b = 5.8949(8) A, c = 5.5852(8) A, Z = 4; 2, monoclinic, space group P2(1)/n, a = 4.3007(3) A, b = 9.5003(7) A, c = 11.5877(9) A, beta = 95.855(1) degrees, Z = 4; 3, triclinic, space group Ponemacr;, a = 7.1066(6) A, b = 8.3503(7) A, c = 8.3554(7) A, alpha = 89.349(1) degrees, beta = 77.034(1) degrees, gamma = 76.561(1) degrees, Z = 2.

Cation-cation interactions in neptunyl(V) compounds: hydrothermal preparation and structural characterization of NpO2(IO3) and alpha- and beta-AgNpO2(SeO3).

We describe the synthesis, solid state and solution properties of two families of uranyl(VI) complexes that are ligated by neutral monodentate and anionic bidentate PO, PNH and AsO ligands bearing pendent phenyl chromophores. The uranyl(VI) ions in these complexes possess long-lived photoluminescent LMCT 3Πu excited states, which can be exploited as a sensitive probe of electronic structure, bonding and aggregation behaviour in non-aqueous media. For a family of well defined complexes of given symmetry in trans-[UO2Cl2(L2)] (L = Ph3PO (1), Ph3AsO (2) and Ph3PNH (3)), the emission spectral profiles in CH2Cl2 are indicative of the strength of the donor atoms bound in the equatorial plane and the uranyl bond strength; the uranyl LMCT emission maxima are shifted to lower energy as the donor strength of L increases. The luminescence lifetimes in fluid solution mirror these observations (0.87–3.46 μs) and are particularly sensitive to vibrational and bimolecular deactivation. In a family of structurally well defined complexes of the related anion, tetraphenylimidodiphosphinate (TPIP), monometallic complexes, [UO2(TPIP)(thf)] (4), [UO2(TPIP)(Cy3PO)] 5), a bimetallic complex [UO2(TPIP)2]2 (6) and a previously known trimetallic complex, [UO2(TPIP)2]3 (7) can be isolated by variation of the synthetic procedure. Complex 7 differs from 6 as the central uranyl ion in 7 is orthogonally connected to the two peripheral ones viauranyl → uranium dative bonds. Each of these oligomers exhibits a characteristic optical fingerprint, where the emission maxima, the spectral shape and temporal decay profiles are unique for each structural form. Notably, excited state intermetallic quenching in the trimetallic complex 7 considerably reduces the luminescence lifetime with respect to the monometallic counterpart 5 (from 2.00 μs to 1.04 μs). This study demonstrates that time resolved and multi-parametric luminescence can be of value in ascertaining solution and structural forms of discrete uranyl(VI) complexes in non-aqueous solution.

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Probing the local coordination environment and nuclearity of uranyl(VI) complexes in non-aqueous media by emission spectroscopy

The potential of using optical spectroscopic techniques, such as Raman and Visible/Near Infrared (Vis/NIR), for on-line process control and special nuclear materials accountability applications at a spent nuclear fuel reprocessing facility was evaluated. Availability of on-line real-time techniques that directly measure process concentrations of nuclear materials will enhance performance and proliferation resistance of the solvent extraction processes. Further, on-line monitoring of radiochemical streams will also improve reprocessing plant operation and safety. This report reviews current state of development of the spectroscopic on-line monitoring techniques for such solutions. To further examine applicability of optical spectroscopy for monitoring reprocessing solutions, segments of a spent nuclear fuel, with approximate burn-up values of 70 MWd/kgM, were dissolved in concentrated nitric acid and adjusted to varying final concentrations of HNO3. The resulting spent fuel solutions were batch-contacted with tributyl phosphate/dodecane organic solvent. The feed and equilibrium aqueous and loaded organic solutions were subjected to optical measurements. The obtained spectra showed the presence of the quantifiable Raman bands due to NO3- and UO22+ and Vis/NIR bands due to multiple species of Pu(IV), Pu(VI), Np(V), the Np(V)-U(VI) cation-cation complex, and Nd(III) in fuel solutions, justifying spectroscopic techniques as a promising methodology for monitoring spent fuel processing solutionsmore » in real-time. Quantitative evaluation of the fuel solution was performed based on spectroscopic measurements and compared to ICP-MS analysis.« less

Spectroscopic monitoring of spent nuclear fuel reprocessing streams: An evaluation of spent fuel solutions via Raman, visible, and near-infrared spectroscopy

“Cation-cation” complexes of pentavalent actinides—I: Spectrophotometric study of complexes between neptunium (V) and UO22+ and NpO22+ ions in aqueous perchloric and nitric solutions

J.P. Moulin

Papers

“Cation-cation” complexes of pentavalent actinides—I: Spectrophotometric study of complexes between neptunium (V) and UO22+ and NpO22+ ions in aqueous perchloric and nitric solutions