Structural chemistry of tetrachloride complexes of uranyl (review)

The crystal structures of tetrachloride complexes of uranyl with cations of alkali metals, and ammonium, protonated cations of organic bases, ammonium derivatives and a number of protonated organic compounds studied by single-crystal X-ray diffraction analysis are systematized and discussed. The crystal chemical features of the structure of this class of compounds have been determined. In the investigated crystal structures of UO₂²⁺ tetrachloride complexes, the coordination polyhedron of the hexavalent uranium atom has a tetragonal-bipyramidal (distorted octahedral) structure with oxygen atoms of the uranyl group located at the apical vertices of the tetragonal-bipyramidal polyhedron. The UO₂²⁺ groups in the coordination polyhedra are directed perpendicular to the equatorial plane, at the vertices of which four Cl atoms of coordinated ligands are located. In the presence in the structures of uranyl tetrachloride complexes, along with complex anions [UO₂Cl₄]^2– and protonated cations, free Cl^– ions and free, not associated with the U atom, neutral or coordinated molecules, containing acceptor atoms, the cations do not interact with the donor atoms of the Cl^– anion, and form hydrogen bonds with the free chlorine ions and acceptor atoms of the free molecules. Due to the high symmetry (almost D_4h) of the complex anion in the structures of tetrachloride complexes of uranyl, the compounds have been widely used for numerous physical studies, in particular spectroscopic, luminescence, functional density theory and others.