Uranyl oxalate complexes. Part I. Preparation and crystal and molecular structure of ammonium uranyl trioxalate

The preparation is reported of ammonium uranyl oxalate complexes with UO2 : C2O4 ratios of 1 : 3, 1 : 2, and 1 : 1·5. The crystal structure and single-crystal visible absorption spectrum of the title compound have been determined. Crystals are monoclinic, space group C2/c, a= 10·031(9), b= 11·519(10), c= 14·213(10)Å, β= 101·46(6)°, Z= 4·1287 Observed intensities were measured by diffractometer; the structure was solved by the heavy-atom method and refined to a final R of 0·072. The crystal contains NH4+ and UO2(C2O4)34– ions. The latter have two bidentate oxalate groups forming five-membered rings with U–O 2·43 Å, and one such group forming a four-membered ring with U–O 2·57 Å. This oxalate group has its two ends twisted relative to each other by 37°. The uranyl U–O distance is 1·69 Å, and the geometry around the uranium atom is approximately hexagonal bipyramidal.