Lone electron pair stereoactivity, cation arrangements and distortion of heteropolyhedral sheets in the structures of Tl₂[(UO₂)(AO₄)₂] (A = Cr, Mo)

Two new Tl uranyl compounds, Tl 2 (UO 2 )(MoO 4 ) 2 and Tl 2 (UO 2 )(CrO 4 ) 2 , have been prepared by high-temperature solid-state reaction and hydrothermal synthesis, respectively. The structure of Tl 2 (UO 2 )(MoO 4 ) 2 (orthorhombic, Pca 2 1 , a = 10.977(3), b = 14.004(3), c = 14.041(3) Å, V = 2158.5(8) Å 3 , Z = 8) has been solved by direct methods and refined to R 1 = 0.051 on the basis of 5229 unique observed reflections (| F o | ≥ 4 σ F ). The structure of Tl 2 (UO 2 )(CrO 4 ) 2 (orthorhombic, Pca 2 1 , a = 10.7034(5), b = 13.4252(6), c = 13.9364(6) Å, V = 2002.6(2) Å 3 , Z = 8) has been solved by direct methods and refined to R 1 = 0.035 on the basis of 6330 unique observed reflections (| F o | ≥ 4 σ F ). The compounds are isostructural and are based upon [(UO 2 )( A O 4 ) 2 ] sheets ( A = Cr, Mo) of Ur O 5 pentagonal bipyramids ( Ur = UO 2 2+ , uranyl ion) that share equatorial corners with A O 4 tetrahedra. The sheets are parallel to (001) and are linked by Tl + cations located in the interlayer. Stereoactive lone electron pairs on the Tl + cations form lone pair-containing cavities that form channels parallel to the a axis and are located at the level of y ∼ 0.5. The repulsion of the Tl + cations across the cavities leads to the distortion of the cation arrangements and, as a consequence, distortion of the flexible [(UO 2 )( A O 4 ) 2 ] uranyl molybdate/chromate sheets.