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Two new ammonium uranyl oxalates (NH 4 ) 2 -[UO 2 (C 2 O 4 ) 2 H 2 O]·2H 2 O (1) and (NH 4 ) 2 [(UO 2 ) 2 (C 2 O 4 ) 3 ]· 3H 2 O (2) and one mixed ammonium hydrazinium uranyl oxalate (NH 4 )(N 2 H 5 )[(UO 2 ) 2 (C 2 O 4 ) 3 ]·3H 2 O (3) have been prepared. The crystal structures of the three compounds have been determined by single-crystal X-ray diffraction. The crystal structures were solved by the Patterson method and Fourier difference techniques, and refined by a least square method on the basis of F 2  for all unique reflections. Crystallographic data: 1, hexagonal, space group P6 1 22,  a =8.143(2) Å,  c =36.179(18) Å,  Z =6,  R 1= 0.0268 for 105 parameters with 1223 reflections with  I ≥ 2σ( I ); 2, monoclinic, space group  P 2 1 / n ,  a =8.705(3) Å,  b = 19.454(6) Å,  c =5.5105(18) Å, β=101.741(5)°,  Z =2,  R 1= 0.0314 for 139 parameters with 2059 reflections with  I ≥ 2σ( I ); 3, monoclinic, space group  P 2 1 / n ,  a =8.958(3) Å,  b =19.601(7) Å,  c = 11.007(4) Å, β=112.928(5)°,  Z =4,  R 1=0.0363 for 131 parameters with 3346 reflections with  I ≥ 2σ( I ). The crystal structure of 1 contains [UO 2 (C 2 O 4 ) 2 · H 2 O] 2−  anions, NH 4  +  cations and water molecules. In the anion, the linear uranyl group is coordinated by two bidentate oxalate anions and one water molecule giving a pentagonal bipyramidal environment for the uranium atom. The structures of 2 and 3 consist of two-dimensional networks built from uranyl and oxalate ions forming hexagonal rings. The oxalate ions are tetradentate, each bridging two uranyl ions which are coordinated by three oxalate groups to give distorted hexagonal bipyramidal environments for the uranium atoms. In 3, the ammonium and hydrazinium ions are ordered in successive layers. The various possible coordinations for the uranyl cation and the great versatility of the oxalate anion are magnificently illustrated in the monovalent ion-uranyl double oxalates series and other structures are expected by the diversification of the synthesis methods and experimental conditions.

radiochimica acta

Crystal structure of new uranyl oxalates (NH 4 ) 2 [UO 2 (C 2 O 4 ) 2 · H 2 O]·2H 2 O and (NH 4 ) 2-x (N 2 H 5 ) x [(UO 2 ) 2 (C 2 O 4 ) 3 ]·3H 2 O (x=0 and x=1). Comparison with other uranyl oxalates