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Three uranyl chromate hydrates, (UO 2 )(CrO 4 )(H 2 O) 2 , [(UO 2 )(CrO 4 )(H 2 O) 2 ](H 2 O) and [(UO 2 )(CrO 4 )(H 2 O) 2 ] 4 (H 2 O) 9 , have been prepared by evaporation from aqueous solutions. The crystal structure of (UO 2 )(CrO 4 )(H 2 O) 2  (monoclinic,  C 2/ m ,  Z  = 16,  a  = 16.786(2),  b  = 22.731(2),  c  = 6.9969(7) Å,  β  = 90.051(3)°,  V  = 2669.8(5) Å 3 ) has been solved by direct methods and refined to  R 1 = 0.065, calculated for 6702 unique observed reflections (| F  o | ≥ 4 σ F  ). The crystal structure of [(UO 2 )(CrO 4 )(H 2 O) 2 ](H 2 O) (monoclinic,  P 2 1 ,  Z  = 4,  a  = 9.7206(14),  b  = 7.1617(10),  c  = 11.0909(16) Å,  β  = 92.388(3)°,  V  = 771.43(19) Å 3 ) has been solved by direct methods and refined to  R 1 = 0.052, calculated for 4879 unique observed reflections (| F  o | ≥ 4 σ F  ). The crystal structure of [(UO 2 )(CrO 4 )(H 2 O) 2 ] 4 (H 2 O) 9  (monoclinic,  P 2 1 / c ,  Z  = 4,  a  = 31.397(1),  b  = 7.1701(3),  c  = 16.2480(7) Å,  β  = 97.515(1)°,  V  = 3626.3(3) Å 3 ) has been solved by direct methods and refined to  R 1 = 0.051, calculated for 8164 unique observed reflections (| F  o | ≥ 4 σ F  ). Each structure contains chains of composition [(UO 2 )(CrO 4 )(H 2 O) 2 ]. Within the chains, (UO 2 )O 3 (H 2 O) 2  pentagonal bipyramids share three equatorial O atoms with CrO 4  tetrahedra. The CrO 4  tetrahedra in the [(UO 2 )(CrO 4 )(H 2 O) 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 ) ∞ ]. The structures of (UO 2 )(CrO 4 )(H 2 O) 2  and [(UO 2 )(CrO 4 )(H 2 O) 2 ](H 2 O) are based upon the chains of the ( ud ) ∞  type, whereas [(UO 2 )(CrO 4 )(H 2 O) 2 ] 4 (H 2 O) 9  contains two types of chains: ( ud ) ∞  and ( u ) ∞ . The [(UO 2 )(CrO 4 )(H 2 O) 2 ] chains extend in the same direction in each structure and are linked via hydrogen bonds involving H 2 O molecules.

zeitschrift für kristallographie - crystalline materials

Geometrical isomerism in uranyl chromates I. Crystal structures of (UO 2 )(CrO 4 )(H 2 O) 2 , [(UO 2 )(CrO 4 )(H 2 O) 2 ](H 2 O) and [(UO 2 )(CrO 4 )(H 2 O) 2 ] 4 (H 2 O) 9