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

Abstract

Abstract 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.