Synthesis and crystal structure of cesium actinide(VI) tricarbonate complexes Cs4AnO2(CO3)3·6H2O, An(VI) = U, Np, Pu

Abstract

Tricarbonate complexes of hexavalent U, Np, and Pu with outer-sphere cesium cations, Cs₄AnO₂·(CO₃)₃·6H₂O, were synthesized and studied by single crystal X-ray diffraction analysis. Crystals of Cs₄AnO₂·(CO₃)₃·6H₂O consist of [AnO₂(CO₃)₃]^4– complex anions and hydrated Cs⁺ cations. The coordination polyhedron (CP) of An(VI) atoms is a distorted hexagonal bipyramid with three CO₃^2– anions arranged in the equatorial plane. Four independent Cs+ cations have the coordination surrounding in the form of 11-, 10-, and 9-vertex polyhedra formed by the O atoms of CO₃^2– anions, AnO₂²⁺ cations, and water molecules. Six crystallographically independent water molecules in the structure of Cs₄AnO₂(CO₃)₃·6H₂O form a three-dimensional system of hydrogen bonds in which the O atoms of carbonate ions and water molecules act as proton acceptors. The “yl” oxygen atoms of AnO₂²⁺ cations are not involved in hydrogen bonding. The lengths of the An–O_carb bonds in the equator of the U, Np, and Pu hexagonal bipyramids are noticeably influenced by incorporation of the O atoms of the CO₃^2– anions in the coordination polyhedra of Cs⁺ ions and by involvement of these atoms in hydrogen bonding.