Insights about the redox speciation of neptunium in an aqueous mineral acid electrolyte were obtained through a combination of in situ EXAFS (extended X-ray absorption fine structure) spectroelectrochemistry, density functional theory (DFT), and simple geometric modeling. A single solution of neptunium in 1 M perchloric acid was used to extract metrical information about the Np coordination environment, in terms of hydration numbers (n) and Np-O interatomic distances. Four aquo ions - Np

The alpha-1 and alpha-2 isomers of the monovacant Wells-Dawson heteropolyoxoanion [P(2)W(17)O(61)](10-) are complexants of trivalent rare-earth (RE) ions and serve to stabilize otherwise reactive tetravalent lanthanide (Ln) and actinide (An) ions in aqueous solution. Aspects of the bonding of Ln ions with alpha-1-[P(2)W(17)O(61)](10-) and alpha-2-[P(2)W(17)O(61)](10-) were investigated to address issues of complex formation and stability. We present structural insights about the Ln(III) coordination environment and hydration in two types of stoichiometric complexes, [Ln(alpha-1-P(2)W(17)O(61))](7-) and [Ln(alpha-2-X(2)W(17)O(61))(2)](17-) (for Ln identical with Sm, Eu, Lu; X identical with P, As). The crystal and molecular structures of [(H(2)O)(4)Lu(alpha-1-P(2)W(17)O(61))](7-) (1) and [Lu(alpha-2-P(2)W(17)O(61))(2)](17-) (2) were solved and refined through use of single-crystal X-ray diffraction. The crystallographic results are supported with corresponding insights from XAFS (X-ray absorption fine structure) for a series of nine solid-state complexes as well as from optical luminescence spectroscopy of the Eu(III) analogues in aqueous solution. All the Ln ions are eight-coordinate with oxygen atoms in a square antiprism arrangement. For the 1:1 stoichiometric Ln/alpha-1-[P(2)W(17)O(61)](10-) complexes, the Ln ions are bound to four O atoms of the lacunary polyoxometalate framework in addition to four O atoms from solvent (water) molecules as [(H(2)O)(4)Ln(alpha-1-P(2)W(17)O(61))](7-). This structure (1) is the first of its kind for any metal complex of alpha-1-[P(2)W(17)O(61)](10-), and the data indicate that the general stoichiometry [(H(2)O)(4)Ln(alpha-1-P(2)W(17)O(61))](7-) is maintained throughout the lanthanide series. For the 1:2 stoichiometric Ln/alpha-2-[X(2)W(17)O(61)](10-) complexes, no water molecules are in the Ln-O(8) coordination sphere. The Ln ions are bound to eight O atoms-four from each of two heteropolyanions-as [Ln(alpha-2-X(2)W(17)O(61))(2)](17-). The average Ln-O interatomic distances decrease across the lanthanide series, consistent with the decreasing Ln ionic radius.

The coordination environment of Np(VII) in solution remains uncertain despite numerous studies. Interest in the heptavalent Np ion is driven by the implications of the coordination environment on the basic understanding of structure/bonding relationships and its technological importance in treating nuclear waste. Initially reported by Krot and Gelman in 1967, 1 this f 0 configuration of Np is slowly reduced by H 2 O in strongly alkaline solutions, and rapidly reduced in acidic media. Studies in highly basic solutions have reported various coordination geometries, including the dioxo moiety, OdNpdO, coordinated equatorially to oxo or hydroxo anions or waters, 2,3 and a square planar tetraoxo complex with two more distant axial hydroxide ions. 4 The latter Np(VII) coordination has precedent in the solid state. 5 Analogies with other high-valent actinide ions, including U(VI), Np(VI), and Pu(VI), suggest that the hydroxy neptunyl ion NpO 2 (OH) n (3-n) (H 2 O) m is the prevalent species in solution. In contrast, the oxo anions of hexa-and heptavalent transition metals exhibit tetrahedral MO 4 ncoordination. We report the results of Np L 3 -edge X-ray absorption spectroscopy (XAS) experiments optimized to determine the coordination environments of Np(VI) and Np(VII) in a highly basic solution. The results of these experiments, taken together with density functional theory (DFT) calculations, show Np(VII) to have a tetraoxo coordination with two additional more distant hydroxy ligands or water molecules.An alkaline suspension of Np was prepared by precipitating Np(V) with concentrated NaOH. The resulting Np(V) hydroxide precipitate was slurried with 1 M NaOH and transferred to a purpose-built spectroelectrochemical cell equipped with Pt working and auxiliary electrodes and a Ag/AgCl reference electrode. 6 Np(V) was oxidized, first to Np(VI) and then to Np(VII), by continuously sparging the solution with ozone while simultaneously applying an oxidizing potential of +600 mV vs Ag/AgCl. The standard reduction potential for Np(VII) under the conditions of our experiment has been previously reported as +384 mV. 7 After several hours, the ozone was stopped and the applied potential was reduced to +500 mV. X-ray data were collected 8 on the oxidized Np. The applied potential was reduced and data were obtained after bulk electrolysis at intermediate potentials until the solution contained only Np(VI).The L 3 -edge XANES data obtained from the 6.5 mM Np solution during and after ozonolysis and as a function of applied potential are shown in Figure 1. The Np(VI) spectrum, part d in Figure 1, exhibits the shoulder feature characteristic of an actinyl moiety, 9,10 and is similar to the spectrum of Np(VI) and Np(V) in aqueous acidic solution. 11 In contrast, the spectrum of the Np(VII) species has a broader, doubled white line, thereby suggesting a change in the coordination environment from a simple OdNpdO configuration. The coordination environments of Np(VI) and Np(VII) are quantified by analyses of the EXAFS data and their Fou...