[en] The complexes of Np(IV) and U(IV) with unsaturated heteropolyanion SiW₁₁O₃₉^8- have been prepared in single crystal form, their structures have been determined by X-ray structural analysis. Actinide atoms in [An(SiW₁₁O₃₉)₂]^12- complex anions are eight-coordinated. Their coordination polyhedra can be described as distorted square anti-prisms. The main distortion of the coordination polyhedron in both complexes is twisting of one base of the square anti-prism from its ideal position by about 9 angle around the fourfold inversion axis. (authors)

[en] After having recalled the principle of the X absorption spectroscopy, the authors give examples illustrating the analytical possibilities of this technique and the different application fields concerning the actinides physico-chemistry (coordination chemistry, interface, solid state, solution). (O.M.)

[en] The topics related to actinide chemistry involved in the civil nuclear industry follow the electronuclear cycle: mining-enrichment, physical chemistry of the nuclear fuel, behavior in reactors, selective extraction for fuel reprocessing, storage, environmental impact, and human toxicology. In order to characterize the actinide properties, dedicated synchrotron beam lines are necessary, although scarce. This article focuses on synchrotron studies related to the elaboration of the nuclear fuel, its reprocessing, the storage in glass matrices and the bio-environmental impact. (author)

[en] It is well known that new three-dimensional molecular structures can be obtained through the use of well-characterised building blocks. Such methods have been widely used to obtain pre-organised three-dimensional entities. Among possible building blocks, the hexa-cyano metallate ions [M(CN)₆]^n- are often used to form polymeric inorganic networks such as Prussian Blue Fe₄[Fe(CN)₆]₃. 15 H₂O. Recently, the possibility to form An[M(CN)₆] solids has been evidenced. The structural study of An[M(CN)₆] complexes is of great interest, all the more because of their possible applications in nuclear fuel reprocessing. As microcrystalline materials were obtained, X-ray powder diffraction (XPD), infrared (IR) and X-ray absorption (XAS) spectroscopies were of particular help to probe the An and Fe environments. We investigated the long-range order using XRPD. The IR vibration bands of the cyano groups probe the chemical bonds of the carbon and nitrogen atoms in these new materials. XAS provides both information about the electronic structure via X-ray absorption near edge spectroscopy (XANES) and about the local structure around the metals via extended X-ray absorption fine structures spectroscopy (EXAFS). Moreover, the evolution of the properties of these new complexes along the actinide series is an indirect probe of the actinide bonding. (authors)

[en] The comparison of the physicochemical behavior of the actinides with that of the lanthanides can be justified by the analogy of their electronic structure, as each of the series is made up of elements corresponding to the filling of a given (n)f atomic shell. However relatively few points of comparison are available, given the lack of available structure for trans-plutonium(III) elements and the additional difficulty of stabilizing coordination complexes of uranium(III) to plutonium(III). This contribution is a focal point of trans-plutonium(III) chemistry and, more specifically, of some americium compounds that have been recently synthesized, all related with hard acid oxygen donor ligands that may be involved in the reprocessing chain of nuclear fuel. After a brief review of the solid hydrates and aquo species for the lanthanide and actinide families, we discuss two types of ligands that have in common three carboxylic groups, namely the amino-tri-acetic acid and the citric acid anions. The additional roles of the nitrogen atom for the first one and of the hydroxy function for the second one are discussed. Accordingly, five new complexes with either americium or lanthanides elements are described: [Co(NH₃)₆][M(NTA)₂(H₂O)].8H₂O with M Nd, Yb and Am, and [Co(NH₃)₆]₂K[M₃(Cit)₄(H₂O)₃].18H₂O with Nd and Am cations. In all cases the americium complexes are isostructural with their lanthanide equivalents. (authors)

[en] The polymeric complex [(NpO₂Py₅)(KI₂Py₂)]_n is prepared from dry 'NpO₂Cl' by anion exchange with potassium iodide in pyridine affording the first convenient starting material for the development of NpO₂⁺ coordination chemistry in anhydrous organic media. (authors)

[en] XANES spectroscopy has long been used as a structural and electronic probe of a selected element. Phenomenological application of this technique to actinide cations has proved fruitful to characterize the actinide environment in both solid state and solution compounds. Although powerful XANES simulation codes have been developed, the use of such simulations in order to describe the valence orbitals of the actinide cation is still scarce. The very short life time of the core hole at the LIII edge as well as the low symmetry and large size of the coordination polyhedron are difficulties to be overcome in the analysis of the edge spectra. In this work, three simple molecules have been selected for their similar geometry that is typical of the trans di-oxo actinyl compounds: [UO₂(H₂O)₅]²⁺, [NpO₂(H₂O)₅]²⁺, [NpO₂(OH)₄]^2-. Additional comparison with a transition metal, the osmyl cation [OsO₂(OH)₄]^2-, is also made. The cation LIII edges have been recorded and compared to edge calculations using FEFF8.2 code. This article is structured in two parts. In the first one, elaboration and optimization of a valid structural model cluster is carried out using molecular dynamics calculations. The influence of the water solvent molecules as well as the hydrogen atoms of the cations first coordination sphere are discussed. In the second part, Amsterdam quantum chemical calculations have been carried out on the four clusters and molecular energy levels are qualitatively compared to the data obtained from calculated XANES spectra. (authors)

[en] The title compound, (C₁₆H₃₆N)₃[Th(NCS)₄(NO₃)₃], was obtained from the reaction of Th(NO₃)₄.5H₂O with (Bu₄N)(NCS). The Th(IV) atom is in a ten-coordinate environment of irregular geometry, being bound to the N atoms of the four thiocyanate ions and to three bidentate nitrate ions. The average Th-N and Th-O bond lengths are 2.481 (10) and 2.57 (3) Angstroms, respectively. (authors)

[en] In summary, the first MC simulation of the trivalent cation of californium, based on an exchangeable hydrated ion-water intermolecular potential, has been shown to extend and improve the hydrated ion model. Likewise, the CfL_III-edge EXAFS spectrum of an acidic 1 mm Cf(ClO₄)₃ aqueous solution recorded under optimized experimental conditions has greatly improved the signal/noise ratio of the only previously recorded spectrum. The comparison of the experimental EXAFS spectrum with the two computed ones, obtained from two different intermolecular potentials that predict eight (BP86) or nine (MP2) water molecules in the first coordination shell, leads to the conclusion that the lowest hydration number is preferred. Then, as Cf^III is the heaviest actinide aqua ion for which there is experimental information, the actinide contraction is supported by the present study. (For U^III, R_U-O=2.56 Angstroms, and CN=9±1; for Pu^III, R_Pu-O=2.51 Angstroms and CN=9±1; for Cm^III, R_Cm-O=2.47 Angstroms and CN=9±1). The role of the second hydration shell is important in defining the structure and dynamics of the Cf^III aqua ion, but the contribution of second-shell water molecules to the EXAFS signal as back-scatters is marginal. Finally, this work gives an illustrative example of the benefits which can be achieved from the combination of experimental X-ray absorption spectroscopy and computer simulations. The usefulness of the simultaneous analysis of the results as well as the importance of the structural statistical average has been clearly demonstrated herein. Each technique independently was not adequate. We believe that this study traces out a still poorly explored combined methodology which may be extremely useful for many other complexes and chemical problems. A systematic theoretical and experimental examination of the other known actinide cations on the same basis should be undertaken to confirm the findings presented herein. (authors)

[en] Although transition metal cyano bimetallic compounds have been well known for decades for their interesting optical and magnetic properties, reports on actinide hexacyanoferrate compounds are scarce. For instance, a thorough structural description is still lacking. Another question is the possible covalency or charge transfer effects in these materials that are known to foster electron delocalization with a large variety of transition metal cations. In this paper, new members of the actinide(IV) hexa-cyanoferrates have been synthesized with Th, Np and Pu. This is the first review of thorium to plutonium hexacyanoferrate compounds since the early investigations during the Manhattan Project some 70 years ago. We have carried out an extensive structural characterization using powder X-ray Diffraction (XRD), X-ray Absorption Spectroscopy (XAS) and X-ray microscopy for the plutonium adduct. The crystallographic space group of microcrystalline Th, Np and Pu hexacyanoferrate compounds appears to be very similar to that of the early lanthanide adducts, suggesting that the tetravalent actinides are arranged in a tri-capped trigonal prismatic polyhedron of coordination number 9, in which the actinide atom is bonded to six nitrogen atoms and to three water molecules. Further combined analysis of the iron K-edge and actinide LIII-edge EXAFS data and XRD data provided the basis for a three dimensional molecular model. Structural data in terms of actinide-ligand bond lengths have been compared to those reported for the parent lanthanide(III) compounds, confirming the structural similarities. In addition, two new structures with the thorium cation have been obtained and described using single-crystal XRD: (H₅O₂)[Th(DMF)₅(H₂O)]₂[Fe(CN)₆]₃ and [Th(DMF)₄(H₂O)₃][Fe(CN)₆](NO₃)2H₂O. This structural description of the Th, Np and Pu hexacyanoferrate system will be followed by a semi-quantitative electronic description of the actinide-cyano bond using NEXAFS data analysis in a coming paper. (authors)