Structural features of two polymorphs of ammonium uranyl crotonate

X-ray crystallography, FTIR spectroscopy and thermal analysis were conducted for crystals of two polymorphs of NH4UO2(C3H5COO)3, which are the first examples of uranyl tricarboxylate complexes with ions of unsaturated aliphatic monocarboxylic acid. The main structural units of both polymorphs are anionic complexes [UO2(C3H5COO)3]−. Uranium atoms adopt hexagonal bipyramidal coordination polyhedron, and crotonate ions realize bidentate cyclic type of coordination. Due to different packing of ammonium ions and [UO2(C3H5COO)3]− complexes triclinic crystals of α-polymorph have layer structure and cubic crystals of β-polymorph have three dimensional structure. Features of three dimensional packing of R+ ions and [UO2L3]− complexes in structures of 25 known R[UO2L3] compounds are discussed.