Monomeric, dimeric and polymeric structure of the uranium trichloride hydrates

Three different uranium trichloride hydrates of the formulas UCl3 · 2H2O · 2CH3CN, UCl3 · 6H2O and UCl3 · 7H2O have been isolated from methylcyanide solutions of UCl4, with a 1–5% water content, by reduction with liquid zinc amalgam. The crystal and molecular structures of the hydrates have been determined by X-ray single crystal analysis. UCl3 · 2H2O · 2CH3CN crystallizes in the triclinic system (space group P 1 ¯ ) forming infinite zigzag chains in the [0 1 0] direction. The coordination polyhedron of the uranium atom is a bicapped trigonal prism, formed by two oxygen atoms of the water molecules, four bridging chlorine atoms, one terminal chlorine atom and one nitrogen atom of the methylcyanide molecule. UCl3 · 6H2O crystallizes in the monoclinic system (space group P2/n), with a monomeric structure. The uranium ion is eight-coordinated by two chlorine atoms and six oxygen atoms of the water molecules, which form a distorted square antiprism. UCl3 · 7H2O crystallizes in the triclinic system in the space group P 1 ¯ . The characteristic feature of this structure are [(H2O)7UCl2U(H2O)7]4+ dimers. The coordination polyhedra, in the form of a tricapped trigonal prism, are linked through the complex network of the hydrogen bonds. UCl3 · 6H2O and UCl3 · 7H2O are isostructural with their lanthanide analogues.