Oleum and Sulfuric Acid as Reaction Media: The Actinide Examples UO 2 (S 2 O 7 )‐lt (low temperature), UO 2 (S 2 O 7 )‐ht (high temperature), UO 2 (HSO 4 ) 2 , An(SO 4 ) 2 (An = Th, U), Th 4 (HSO 4 ) 2 (SO 4 ) 7 and Th(HSO 4 ) 2 (SO 4 )

New uranium sulfates have been synthesised by the reaction of UO 2 (CH 3 COO) 2 · 2H 2 O with sulfuric acid/sulfur trioxide mixtures in sealed glass ampoules. Both polymorphs of the disulfate UO 2 (S 2 O 7 ) [low temperature (lt): triclinic, P $\bar {1}$ , Z = 2, a = 647.8(1) pm, b = 684.2(1) pm, c = 800.1(1) pm, α = 87.31(2)°, β = 78.37(2)°, γ = 75.34(2)°, V = 0.33605(9) nm 3 ; high temperature (ht): orthorhombic, Pca 2 1 , Z = 8, a = 1080.21(2) pm, b = 814.74(1) pm, c = 1655.63(3) pm, V = 1.45710(4) nm 3 ] and the hydrogensulfate UO 2 (HSO 4 ) 2 [monoclinic, P 2 1 / n , Z = 4, a = 779(3) pm, b = 815.39(3) pm, c = 1285.77(5) pm, β = 103.700(1)°, V = 0.79428(5) nm 3 ] contain hexavalent uranium in the form of the well‐known linear [UO 2 ] 2+ ion. The pentagonal bipyramidal coordination sphere around U is completed by disulfate and hydrogensulfate ions, which connect the uranyl groups to a three‐dimensional network. Reaction of uranyl acetate with sulfuric acid at sufficiently high temperature (300 °C) leads to a reduction of uranium to the tetravalent form under separation of green plates of U(SO 4 ) 2 [orthorhombic, Pbca , Z = 8, a = 940.11(2) pm, b = 898.13(2) pm, c = 1349.09(3) pm, V = 1.13909(4) nm 3 ]. The U 4+ ions in U(SO 4 ) 2 are in a square antiprismatic coordination of eight monodentate sulfate tetrahedra. By treatment of ThO 2 with sulfuric acid, the thorium (hydrogen)sulfates Th(SO 4 ) 2 [orthorhombic, Pbca , Z = 8, a = 953.74(3) pm, b = 911.49(3) pm, c = 1365.02(4) pm, V = 1.18665(6) nm 3 ], Th 4 (HSO 4 ) 2 (SO 4 ) 7 [tetragonal, I $\bar {4}$ 2 d , Z = 4, a = 1327.7(2) pm, c = 1362.8(2) pm, V = 2.402.3(6) nm 3 ] and Th(HSO 4 ) 2 (SO 4 ) [triclinic, P $\bar {1}$ , Z = 2, a = 576.18(2) pm, b = 896.11(3) pm, c = 948.58(3) pm, α = 65.387(2)°, β = 87.838(2)°, γ = 88.917(2)°, V = 0.44495(3) nm 3 ] could be obtained. Whereas Th(SO 4 ) 2 is isotypic with the respective uranium compound, both hydrogensulfates contain ninefold‐coordinated thorium to form a triple‐capped trigonal prism. The structure of Th(HSO 4 ) 2 (SO 4 ) can be described as a distorted variant of the UCl 3 structure type.