Geometrical isomerism in uranyl chromates I.: Crystal structures of (UO2)(CrO4)(H2O)2, [(UO2)(CrO4)(H2O)2](H2O) and [(UO2)(CrO4)(H2O)2]4(H2O)9

Three uranyl chromate hydrates,(UO2)(CrO4)(H2O)(2), [(UO2KrO4)(H2O)(2)](H2O) and [(UO2)(CrO4)(H2O)(2)](4)(H2O)(9), have been prepared by evaporation from aqueous solutions. The crystal structure of (UO2KrO4)(H2O)(2) (monoclinic, C2/m, Z = 16, a = 16.786(2), b = 22.731(2), c = 6.9969(7) Angstrom, beta = 90.051(3)degrees, V = 2669.8(5) Angstrom(3)) has been solved by direct methods and refined to R1 = 0.065, calculated for 6702 unique observed reflections (\F-o\ greater than or equal to 4sigma(F)). The crystal structure of [(UO2)(CrO4)(H2O)(2)](H2O) (monoclinic, P2(1), Z = 4, a = 9.7206(14), b = 7.1617(10), c = 11.0909(16) Angstrom, beta = 92.388(3)degrees, V = 771.43(19) Angstrom(3)) has been solved by direct methods and refined to R1 = 0.052, calculated for 4879 unique observed reflections (\F-o\ greater than or equal to 4sigma(F)). The crystal structure of [UO2(CrO4)(H2O)(2)](4)(H2O)(9) (monoclinic, P2(1)/c, Z = 4, a = 31.397(1), b = 7.1701(3), c = 16.2480(7) Angstrom, = beta = 97.515(1)degrees, V = 3626.3(3) Angstrom(3)) has been solved by direct methods and refined to R1 = 0.051, calculated for 8164 unique observed reflections (\F-o\ greater than or equal to 4sigma(F)). Each structure contains chains of composition [UO2)(CrO4)(H2O)(2)]. Within the chains, (UO2)O-3(H2O)(2) pentagonal bipyramids share three equatorial O atoms with CrO4 tetrahedra. The CrO4 tetrahedra in the [(UO2)(CrO4)(H2O)(2)] chains each share three corners with uranyl polyhedra, and contain one non-shared (terminal) ligand pointing either up or down relative to the plane of the chain. This results in a geometrical isomerism of the uranyl chromate chains that can be described as a sequence of tetrahedra orientations along the chain extension, e.g. ''... up-down-up-down ..." or ... ududud .... [or (ud)(infinity)]. The structures of (UO2)(CrO4)(H2O)(2) and [(UO2)(CrO4)(H2O)(2)](H2O) are based upon the chains of the (ud)(infinity) type, whereas [(UO2KrO4)(H2O)(2)](4)(H2O)(9) contains two types of chains: (ud)(infinity) and (u)(infinity). The [(UO2(CrO4)(H2O)(2)] chains extend in the same direction in each structure and are linked via hydrogen bonds involving H2O molecules.