A. B. Yusov

Hydrolysis of Np(IV) and Pu(IV) and their complexation by aqueous Si(OH)4

Summary The hydrolysis and interaction of Np(IV) and Pu(IV) with orthosilicic acid, Si(OH)4, were studied in 0.1–1.0 M ionic strength aqueous solutions. Spectrophotometry was used to study these reactions at about 10–4 M Np(IV) and Pu(IV) concentrations. The first hydrolysis constants, Khydr, agree with the majority of earlier spectrophotometric and potentiometric data. The absorption spectra of NpOH3+ and PuOH3+ were obtained by spectral deconvolution. Reasons to explain the overestimation of Khydr obtained by other methods [by extraction of trace amounts of Np(IV) and Pu(IV) and by solubility] are discussed. Formation of the complexes NpOSi(OH)33+ and PuOSi(OH)33+ is demonstrated in the presence of 0.005–0.016 M Si(OH)4 in the p[H+] range 1.0–2.2 and 0.3–1.4, respectively. Measured values of equilibrium constants of the reaction M4+ + Si(OH)4 ↔ MOSi(OH)33+ + H+ at ionic strength I=1.0 are lg K1 = 0.41±0.02 and 1.04±0.04, respectively, for Np(IV) and Pu(IV). The stability constants of the NpOSi(OH)33+ and PuOSi(OH)33+ complexes, recalculated to zero ionic strength, are lg β10 = 11.2 and 11.8, respectively. The correlation between Khydr and K1, as observed for all earlier studied metal ions, also occurs for both Np(IV) and Pu(IV).

Photochemistry of f-element ions

Published data on photochemical reactions of f-element compounds, namely, uranyl ion and lanthanide and actinide ions, are surveyed and analyzed. The types of reactions of photoexcited ions, reaction mechanisms, and analytical applications of the photochemical methods for the separation, isolation, and determination of f-elements are considered.

Reaction of Plutonium(VI) with Orthosilicic Acid Si(OH)4

Reaction of Pu(VI) with Si(OH)4 (at concentration 0.004–0.025 mol l–1) in a 0.2 M NaClO4 solution at pH 3–8 is studied by spectrophotometric method. In the range of pH 4.5–5.5, PuO2(H2O)4OSi(OH)3+ complex is formed, while at pH > 6, PuO2(H2O)3O2Si(OH)2 or hydroxosilicate complex PuO2(H2O)3(OH)OSi(OH)3 is recorded. The equilibrium constants are calculated for the reactions of formation of PuO2(H2O)4OSi(OH)3+ and PuO2(H2O)3O2Si(OH)2 and their concentration stability constants: log K1 = –3.91 ± 0.17 and log K2≈ –10.5; log β1= 5.90 ± 0.17 and log β2≈ 12.6. The PuO2(H2O)4OSi(OH)3+ complex is significantly less stable than analogous complex of U(VI). Calculations of the forms of Pu(VI) occurrence at the Si(OH)4 concentration equal to 0.002 mol l–1 showed that the maximum fraction of the PuO2(H2O)4OSi(OH)3+ complex is ∼10% (pH 6.5), while the fraction of PuO2(H2O)3O2Si(OH)2 is almost 40% (pH 8).

Complexation of An(VI) (An = U, Np, Pu, Am) with 2,6-pyridinedicarboxylic acid in aqueous solutions. Synthesis and structures of new crystalline compounds of U(VI), Np(VI), and Pu(VI)

Complexation of An(VI) (An = U, Np, Pu, Am) with 2,6-pyridinedicarboxylic (dipicolinic) acid in aqueous solutions was studied. All these actinides form with dipicolinic acid anion, PDC2− 1: 1 and 1: 2 complexes. The PDC2− ion coordinates to actinide(VI) ions in solutions in tridentate fashion. In 1: 2 complexes, the f-f transition bands in the electronic absorption spectra are very weak, which is associated with approximate central symmetry of the coordination polyhedron (CP) of the An atom. The apparent stability constants of Pu(VI) complexes were measured in a wide pH range, and the concentration stability constants of An(VI) (An = U, Np, Pu, Am) were determined. The crystalline complexes [Li2AnO2(PDC)2]·2H2O (An = U, Np, Pu) and [AnO2(PDC)]n (An = Np, Pu) were synthesized, and their structures were determined by single crystal X-ray diffraction. The X-ray data confirmed the conclusion that CP of An atoms in the complex ions AnO2·(PDC)22− is centrosymmetrical. In the isostructural series of [Li2AnO2(PDC)2]·2H2O, the actinide contraction is manifested in shortening of the An-O distances in the “yl” groups in going from U to Pu.

Synthesis and structure of crystalline complexes of Np(V) with 1,10-phenanthroline-2,9-dicarboxylic acid. Complexation in solution and spectral studies

Complexes of Np(V) with 1,10-phenanthroline-2,9-dicarboxylic acid, C12H6N2(COOH)2 (H2PDA), of the compositions [(NpO2)2(PDA)(H2O)3]·H2O (I), (NH4)2[NpO2(PDA)]2·3H2O (II), and [C(NH2)3]2[NpO2·(PDA)]2·4H2O (III) were synthesized. The Np atoms in the crystal lattices of all the compounds have the pentagonal bipyramidal coordination surrounding, with the [C12H6N2(COO)2]2− anions acting as chelate-bridging N,O-donor ligands. In the structure of I, two crystallographically independent NpO2+ dioxocations participate in the cation-cation interaction leading to the formation of tetrameric cation-cation complexes. The nonequivalence of the Np atoms is manifested in splitting of the main absorption band of Np(V) in the electronic spectrum of solid compound I. The structures of II and III are based on dimeric anionic complexes [NpO2(C12H6N2(COO)2)]22−. Only one kind of complexes, NpO2(PDA)−, was detected in the solution, and high value of the concentration stability constant β, ∼1012 L mol−1, is due to tetradentate coordination of the ligand.

Crystal structure of a new neodymium hexamolybdotellurate, Nd2TeMo6O24 · 19H2O

The crystal structure of a new compound, namely, Nd2TeMo6O24 · 19H2O, is determined using X-ray diffraction. The crystal has a chain structure and consists of [Nd2TeMo6O24 · 14H2O]n neutral chains aligned parallel to the [010] direction and crystallization water molecules. In a chain, each Nd atom links two heteropoly anions. The Nd3+ environment includes seven water molecules and two oxygen atoms of the two heteropoly anions adjacent in the chain. The polyhedron is a monocapped tetragonal antiprism. In the previously studied complex of similar composition, namely, Nd2TeMo6O24 · 18H2O, the Nd coordination polyhedron has the shape of a tricapped trigonal prism formed by six water molecules and three oxygen atoms of the two heteropoly anions adjacent in the chain.

Interaction of Aluminium(III) with Uranyl Ions in the Course of Joint Hydrolysis

Interaction of U(VI) with Al(III) in solutions at pH ≥ 2 and in precipitates obtained at pH ≥ 5 was studied using spectrophotometry, luminescence, and IR spectroscopy. It was shown that in the range of pH 3–4, hydrolyzed forms of uranyl and of aluminium come into interaction. The mixed hydroxoaqua complexes (H2O)3UO2(μ-OH)2Al(H2O)3+4,(H2O)3UO2(μ-OH)2Al(OH)(H2O)2+3, or (H2O)4UO2OAl(OH)(H2O)2+4are likely to form in the solution. With an increase in pH, mixed polymers of a large size (oligomers) can form which further take part in the precipitate formation. The reaction between U(VI) and Al(III) in precipitates is confirmed by the data of IR spectroscopy and by the changes in the physico-chemical properties of these precipitates as compared with the properties of a mechanical mixture of separately precipitated uranium and aluminium. The important role of the oligomeric mixed forms in the formation of precipitate remains at pH levels varying from 5 to 14.

Interaction of U(VI), Np(VI), and Pu(VI) with picolinic (2-pyridinecarboxylic) acid: Complexation in aqueous solutions and synthesis, spectra, and thermal properties of complexes [CH6N3][AnO2(C6H4NO2)3] (An = U, Np, Pu)

Complexation of actinides(VI) (U, Np, Pu) with picolinic acid C6H5NO2 (HPic) in solution was studied by spectrophotometry. In particular, data on the Np(VI) complexation were obtained for the first time. The complexes [AnO2(HPic)]2+, [AnO2(Pic)]+, [AnO2(Pic)2], and, presumably, [AnO2(HPic)(Pic)]+, and also mixed picolinate-hydroxide complexes can exist in the solution under different conditions. The stability constants of the complexes were estimated. A series of crystalline actinide(VI) picolinate complexes Gu[AnO2(Pic)3] (An = Np, Pu; Pic is picolinate ion, Gu is guanidinium ion) were synthesized, and their structure was determined. The spectra and thermal behavior of the complexes are discussed.

Interaction of U(VI), Np(VI), Pu(VI), and Np(V) with benzenetricarboxylic acids: Complexation in aqueous solutions, synthesis, and crystal structure of uranyl complexes [UO2(H2O)2{C6H3(COO)2(COOH)}]2·2H2O and [(UO2)3(H2O)4{C6H3(COO)3}2]

The complexation of U(VI), Np(VI), and Pu(VI) and of Np(V) with 1,2,3- and 1,2,4-benzenetricarboxylic acids (BTC) in aqueous solutions was studied in wide ranges of pH and actinyl ion concentrations. The compositions of the forming hexavalent actinide complexes were determined. Their apparent stability constants β1′ depend on pH of the solution: in the pH range 2–4, logβ1′ from 2 to 4 for the complexes of U(VI), Np(VI), and Pu(VI) with 1,2,3-BTC and from 1.5 to 3.5 for the complexes with 1,2,4-BTC. For Np(V), the β1′ values are close with both acids, and at equal pH values the Np(V) complexes are less stable than the An(VI) complexes (An = U, Np, Pu). With an increase in pH from ∼3 to 6.2–6.9, logβ1′ of the Np(V) complexes increases approximately from 0.5 to 3. Solid U(VI) complexes with 1,2,3- and 1,2,4-benzenetricarboxylic acids were synthesized by the hydrothermal method, their crystal structure was determined, and the IR spectra were examined.

Complexation of hexavalent U, Np, and Pu with cyclopropanecarboxylate anions. Crystal structure of the uranyl complexes {[UO2(bipy)(cpc)]2O2} and [UO2(cpc)2(H2O)2] (bipy = 2,2′-Bipyridine, cpc− = cyclopropanecarboxylate anion)

The complexation of hexavalent U, Np, and Pu with cyclopropanecarboxylate anions, cpc−, in aqueous solutions was studied. The stepwise concentration stability constants of the complexes PuO2(cpc)i2−i (i = 1, 2, 3) are as follows: logK1,2,3 = 2.63 ± 0.20, 1.61 ± 0.20, 1.43 ± 0.20, respectively; the overall concentration stability constant of the complex PuO2(cpc)3− is logβ3 = 5.67 ± 0.60. The complexing properties of the cpc− anion are very close to those of butyrate and isobutyrate anions. Two crystalline uranyl compounds were synthesized: {[UO2(bipy)(cpc)]2O2} (bipy = 2,2′-bipyridine) and [UO2(cpc)2(H2O)2]. The specific feature of the first complex is that it contains peroxide ion. Its appearance may be due to the formation of the cationic moiety via hydrolytic uranyl dimer. The second compound forms a 3D structure, with the complexes linked via hydrogen bonds.