Stereochemistry of Neptunium in Oxygen-Containing Compounds

Coordination of Np(III–VII) atoms in the crystal structures of all the oxygen-containing compounds characterized with the R-factor lower than 0.1 was analyzed with the aid of Voronoi–Dirichlet polyhedra (VDPs). Nine types of NpO _n coordination polyhedra (6 ≤ n ≤ 12) are realized. The most characteristic of them are trigonal dodecahedra [Np(IV)], penta- and hexagonal bipyramids [Np(V) and Np(VI)], and octahedra [Np(VII)] based on square NpO₄^– cores. For Np atoms of a fixed oxidation state, the volume of their VDPs in the NpO _n complexes is virtually independent of the coordination number n. The VDP parameters can be used for determining the valence state of the Np atoms, finding compounds with the maximal nonlinearity of the NpO₂⁺ and NpO₂²⁺ dioxocations, and revealing errors in the crystal structure data. Anion–anion interactions involving NpO₄^– and OH^– ions are an important structure-forming factor in Np(VII) compounds. In sublattices consisting of Np atoms only (Np sublattices), the rule of 14 neighbors is fulfilled. Compounds in which binding Np···Np 5f interactions in crystal structures are possible were revealed by analysis of the VDPs of the atoms in the Np sublattices. In such compounds, the metal atoms form bent Np=O–Np bridging fragments and the Np^VO₇ bipyramids are combined in dimers sharing a common axial edge, with the Np atoms of the dimers being also bound via two carboxylate bridges.     

Synthesis and Structure of U(VI), Np(VI), and Pu(VI) 2-Fluorobenzoates

The compounds NH₄[AnO₂(C₆H₄FCOO₃], where An = U (I), Np (II), or Pu (III), CgH₄COO^? is the 2-fluorobenzoate anion, were synthesized and studied by single crystal X-ray diffraction. Compounds I–III are isostructural and crystallize in the cubic system, space group P2₁3, Z = 4. The main structural units of I–III are mononuclear complexes [AnO₂(C₆H₄COO)₃]^? belonging to crystal-chemical group AB₃¹ (A = AnO₂²⁺, B⁰¹ = C₆H₄FCOO^?). The actinide contraction in the structures of I–III is manifested in a regular decrease in the lengths of the An=0 bonds in the AnO₂²⁺ cations and in the volumes of the Voronoi-Dirichlet polyhedra (VDPs) of the An atoms in the series U-Np-Pu. The intermolecular interactions in crystal structures of I–III were analyzed by the method of molecular VDPs.

     

Crystal structure of Rb3[UO2(CH3COO)3]2[UO2(CH3COO)(NCS)2(H2O)]

The structure of Rb₃[UO₂(CH₃COO)₃]₂[UO₂(CH₃COO)(NCS)₂(H₂O)] was studied by single crystal X-ray diffraction. The compound crystallizes in the monoclinic system with the unit cell parameters (at 100 K) a = 18.387(3), b = 16.398(3), c = 12.460(2) Å, β = 92.837(5)^°, V = 3752.4(11) ų, space group C2/c, Z = 4, R = 0.0390. The uranium-containing structural units of the crystals are isolated mononuclear groups [UO₂(CH₃COO)₃]^? and [UO₂(CH₃COO)(NCS)₂(H₂O)]^?, belonging to crystal-chemical groups AB ₃ ⁰¹ and AB⁰¹M ₃ ¹ (A = UO ₂ ²⁺ , B⁰¹ = CH₃COO^?, M¹ = NCS^? and H₂O) of uranyl complexes, respectively. The uranium-containing complexes are linked in a framework by hydrogen bonds and by electrostatic interactions with Rb⁺ cations.     

Stereochemistry of Bk,Cf, and Es in Oxygen-Containing Compounds

Specific features of the coordination of the Bk, Cf, and Es atoms in the crystal structures of oxygencontaining compounds were considered using the Voronoi–Dirichlet polyhedra (VDPs). These actinides (An) form coordination polyhedra AnO_n (6 ≤ n ≤ 9) of five types. At a fixed An oxidation state, the An VDP volume is virtually independent of the coordination number n. Some parameters of the atomic VDPs in the An sublattices were characterized, and compounds in which binding An–An 5f interactions occur in the crystal structures were revealed. As demonstrated by the example of compounds containing coordination polyhedra AnO₉ (An = U–Cf) in the form of tricapped trigonal prisms, the VDP parameters can be used for evaluation of the actinide contraction of An(III) atoms in the crystal structures.     

Synthesis and structure of Cs3[UO2(CH3COO)3]2(NCS)·H2O

The compound Cs₃[UO₂(CH₃COO)₃]₂(NCS)·H₂O (I) was synthesized and studied by IR spectroscopy and single crystal X-ray diffraction. Compound I crystallizes in the monoclinic system with the following unit cell parameters: a = 7.8286(9), b = 19.892(2), c = 20.050(2) Å, β = 94.527(2)°, space group P2₁/c, Z = 4, R = 0.0387. The uranium-containing structural units in crystals of I are mononuclear complexes [UO₂(CH₃COO)₃]^? belonging to crystal-chemical group AB ₃ ⁰¹ (A = UO ₂ ²⁺ , B⁰¹ = CH₃COO^?) of uranyl complexes.     

Cation-cation interactions between uranyl(VI) ions

Data on crystalline U(VI) compounds with mutual coordination of uranyl ions in their structure (cation-cation interactions, CCIs) are summarized. The topology and geometry of CCIs, including the characteristics of Voronoi-Dirichlet polyhedra of uranium atoms, are analyzed. 15 types of complexes [UO ₂ ²⁺ ] _n formed by CCIs are revealed. The influence of CCIs on the stereochemical features of U(VI) are considered, and common and distinctive features of CCIs in uranyl(VI) and neptunyl(V) compounds are discussed. Factors favoring the occurrence of CCIs in uranyl compounds are analyzed. The spectroscopic manifestations of CCIs are considered. Data on CCIs in some noncrystalline media are presented. The strength of cation-cation bonds in uranyl complexes is analyzed.     

Synthesis, structure, and properties of [Be(H2O)4][UO2(CH3COO)3]2

Crystals of previously unknown compound [Be(H₂O)₄][UO₂(CH₃COO)₃]₂ were prepared and studied by X-ray diffraction analysis. The compound crystallizes in the tetragonal system, unit cell parameters (at 100 K): a = 10.3647(3), c = 23.4127(8) Å, V = 2515.16(13) ų, space group I4₁/a, Z = 4, R = 0.0194. The structure consists of mononuclear complexes [Be(H₂O)₄]²⁺ and [UO₂(CH₃COO)₃]^? linked with each other by electrostatic interactions and hydrogen bonds formed by water molecules and O atoms of acetate anions. The compound was also studied by methods of thermal analysis and IR spectroscopy.     

A single crystal X-ray diffraction and IR study of (NH4)2[(UO2)C2O4(CH3COO)2]

A single crystal X-ray diffraction study of (NH₄)₂[(UO₂)C₂O₄(CH₃COO)₂] was performed. The compound crystallizes in the monoclinic system; unit cell parameters (at 100 ± 2 K): a = 6.793(2), b = 18.866(6), c = 20.730(7) Å, β = 90.040(7)°, space group P2₁/c, Z = 8, V = 2656.5(14) ų, R = 0.0495. The main structural units of the crystals are mononuclear complexes [(UO₂)C₂O₄(CH₃COO)₂]^2? belonging to crystal-chemical group AB ₃ ⁰¹ (A = UO ₂ ²⁺ , B⁰¹ = CH₃COO^? and C₂O ₄ ^2? ) of uranyl complexes. The uranium-containing anions are linked with ammonium cations by electrostatic interactions and a system of hydrogen bonds. The results of the X-ray diffraction analysis of the compound are well consistent with the IR data.