| Abstract: | Single crystals of α‐ and β‐Mg2[(UO2)3(SeO4)5](H2O)16 have been synthesized by evaporation from an aqueous solution of the ionic components. The structure of α‐Mg2[(UO2)3(SeO4)5](H2O)16 (monoclinic, C2/c, a = 19.544(3), b = 10.4783(11), c = 18.020(3) Å, β = 91.352(12)°, V = 3689.3(9) Å3) has been solved by direct methods and refined to R1 = 0.048 on the basis of 4338 unique observed reflections. The structure of β‐Mg2[(UO2)3(SeO4)5](H2O)16 (orthorhombic, Pbcm, a = 10.3807(7), b = 22.2341(19), c = 33.739(5) Å, V = 7787.2(14) Å3) has been solved by direct methods and refined to R1 = 0.107 on the basis of 3621 unique observed reflections. The structures of α‐ and β‐Mg2[(UO2)3(SeO4)5](H2O)16 are based upon sheets with the chemical composition [(UO2)3(SeO4)5]4‐. The sheets are formed by corner sharing between pentagonal bipyramids [UO7]8‐ and SeO42‐ tetrahedra. In the α‐modification, the [(UO2)3(SeO4)5]4‐ sheets are more or less planar and run parallel to (001). In the structure of the β‐modification, the uranyl selenate sheets are strongly corrugated and oriented parallel to (010). The [Mg(H2O)6]2+ polyhedra reside in the interlayers and provide three‐dimensional linkage of the uranyl selenate sheets via hydrogen bonding. In addition to H2O groups attached to Mg2+ cations, both structures also contain H2O molecules that are not bonded to any cation. The [(UO2)3(SeO4)5]4‐ sheets in the structures of α‐ and β‐Mg2[(UO2)3(SeO4)5](H2O)16 represent two different structural isomers. The sequences of the orientations of the tetrahedra within the sheets can be described by their orientational matrices with their shortened forms ( ddudd □ /uu □ uud ) and ( dd □ dd □ uu □ uu □ /uuduumdduddm ) for α‐ and β‐Mg2[(UO2)3(SeO4)5](H2O)16, respectively. A short review on the isomerism of [(UO2)3(TO4)5]4‐ sheets (T = S, Cr, Se, Mo) is given. |
