Synthesis and Structure of Mixed Cromate–Nitrate Complexes of Hexavalent Actinides, [(CH3)4N][(AnO2)(CrO4)(NO3)] (An = U, Np, Pu)

Abstract

Mixed-anion compounds of the general composition [(CH3)4N][(AnO2)(CrO4)(NO3)], where An = U, Np, and Pu, were synthesized and structurally characterized. The structural motif of these compounds is based on anionic ribbons of the composition [AnO2(NO3)(CrO4)]nn–, consisting of seven-vertex An polyhedra linked via oppositely oriented CrO4 tetrahedra and NO3 groups acting in the An polyhedra as terminal bidentate ligands. Every four anionic ribbons form channels oriented along b-axis. Tetramethylammonium cations linked with the O atoms of the ribbons by hydrogen bonds are located in these channels. Actinide contraction is observed in the series U–Np–Pu. It is manifested in a regular decrease in the interatomic distances in the An polyhedra, in parameters b and c, and in the unit cell volume.