Synthesis and Crystal Structures of α‐ and β‐Mg 2 [(UO 2 ) 3 (SeO 4 ) 5 ](H 2 O) 16

Single crystals of α‐ and β‐Mg 2 [(UO 2 ) 3 (SeO 4 ) 5 ](H 2 O) 16 have been synthesized by evaporation from an aqueous solution of the ionic components. The structure of α‐Mg 2 [(UO 2 ) 3 (SeO 4 ) 5 ](H 2 O) 16 (monoclinic, C 2/ c , a = 19.544(3), b = 10.4783(11), c = 18.020(3) Å, β = 91.352(12)°, V = 3689.3(9) Å 3 ) has been solved by direct methods and refined to R 1 = 0.048 on the basis of 4338 unique observed reflections. The structure of β‐Mg 2 [(UO 2 ) 3 (SeO 4 ) 5 ](H 2 O) 16 (orthorhombic, Pbcm , a = 10.3807(7), b = 22.2341(19), c = 33.739(5) Å, V = 7787.2(14) Å 3 ) has been solved by direct methods and refined to R 1 = 0.107 on the basis of 3621 unique observed reflections. The structures of α‐ and β‐Mg 2 [(UO 2 ) 3 (SeO 4 ) 5 ](H 2 O) 16 are based upon sheets with the chemical composition [(UO 2 ) 3 (SeO 4 ) 5 ] 4‐ . The sheets are formed by corner sharing between pentagonal bipyramids [UO 7 ] 8‐ and SeO 4 2‐ tetrahedra. In the α‐modification, the [(UO 2 ) 3 (SeO 4 ) 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(H 2 O) 6 ] 2+ polyhedra reside in the interlayers and provide three‐dimensional linkage of the uranyl selenate sheets via hydrogen bonding. In addition to H 2 O groups attached to Mg 2+ cations, both structures also contain H 2 O molecules that are not bonded to any cation. The [(UO 2 ) 3 (SeO 4 ) 5 ] 4‐ sheets in the structures of α‐ and β‐Mg 2 [(UO 2 ) 3 (SeO 4 ) 5 ](H 2 O) 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‐Mg 2 [(UO 2 ) 3 (SeO 4 ) 5 ](H 2 O) 16 , respectively. A short review on the isomerism of [(UO 2 ) 3 ( T O 4 ) 5 ] 4‐ sheets ( T = S, Cr, Se, Mo) is given.