Competitive Coordination of the Uranyl Ion by Perchlorate and Water ‐ The Crystal Structures of UO₂(ClO₄)₂·3H₂O and UO₂(ClO₄)₂·5H₂O and a Redetermination of UO₂(ClO₄)₂·7H₂O

By slow evaporation of solutions containing UO 2 (ClO 4 ) 2 and an excess of HClO 4 , single crystals of [UO 2 (ClO 4 ) 2 (H 2 O) 3 ] ( 1 ) and [UO 2 (H 2 O) 5 ](ClO 4 ) 2 ( 2 ) were obtained and their structures were determined. From similar solutions prepared from stoichiometric amounts of UO 3 and perchloric acid, crystals of [UO 2 (H 2 O) 5 ](ClO 4 ) 2 ·2H 2 O ( 3 ) were obtained. The trihydrate (monoclinic, P 2 1 / c , a = 545.44(1) pm, b = 1811.09(5) pm, c = 1032.46(2) pm, β = 90.016(1)°) consists of uranyl ions, which are coordinated by two monodentate perchlorate ions and three water molecules. The pentahydrate (monoclinic, P 2 1 / n , a = 529.35(2) pm, b = 1645.43(6) pm, c = 1480.18(6) pm, β = 99.847(1)°) contains uranyl ions coordinated by five water molecules. The same structural unit can be found in the heptahydrate, whose structure was re‐determined (orthorhombic, Pbcn , a = 920.9(3) pm, b = 1067.9(3) pm, c = 1445.7(3) pm). In this structure, two molecules of water of crystallization are present.