Geometrical isomerism in uranyl chromates II. Crystal structures of Mg₂[(UO₂)₃(CrO₄)₅](H₂O)₁₇ and Ca₂[(UO₂)₃(CrO₄)₅](H₂O)₁₉

Abstract

Geometrical isomerism of uranyl chromate sheets occurs in the structures of Mg₂[(UO₂)₃(CrO₄)₅](H₂O)₁₇ and Ca₂[(UO₂)₃(CrO₄)₅](H₂O)₁₉. Both compounds were prepared by evaporation from aqueous solutions. Compound 1 is orthorhombic, Pbca, a = 19.9206(10), b = 21.0526(10), c = 18.4966(9) Å, V = 7757.1(7) ų; R₁ = 0.050 for 6332 unique reflections with |F_o| ≥ 4σ_F. Compound 2 is monoclinic, P2₁/m, a = 11.0359(5), b = 17.5364(7), c = 11.5056(5) Å, β = 118.178(1)°, V = 1962.8(2) ų; R₁ = 0.037 for 6209 unique reflections with |F_o| = 4σ_F. Both structures contain sheets of composition [(UO₂)₃(CrO₄)₅] involving corner-sharing UO₇ pentagonal bipyramids and CrO₄ tetrahedra. The sheets are parallel to (100) in both structures. Mg²⁺ and Ca²⁺ cations are coordinated by H₂O molecules and are located in the interlayer. The topology of the [(UO₂)₃(CrO₄)₅] sheets in both structures is the same, whereas their geometrical structures are different, and they are geometrical isomers. The sheets differ in the orientations of non-shared corners of chromate tetrahedra relative to the plane of the sheet. In order to describe the geometrical structure of the isomers, the concept of orientation matrix of tetrahedra is introduced. It has been shown that there are at least three geometrical isomers of the [(UO₂)₃(CrO₄)₅] sheet in uranyl chromates that differ in their orientation matrices of the tetrahedra.