Synthesis and spectra of Np(V) γ-Octamolybdates of the composition M6[(NpO2)2(Mo8O28)]·2H2O (M = NH4, K, Rb, Cs, Tl)

Abstract

Previously unknown Np(V) compounds with γ-octamolybdate anions are reported. A series of complexes of the general composition M6[(NpO2)2(Mo8O28)]·2H2O (M = NH4, K, Rb, Cs, Tl) were synthesized by the hydrothermal method from aqueous solutions. The composition and crystal structure of the compounds were determined by single crystal X-ray diffraction. All the compounds contain the NpO2+ dioxocations, (Mo8O28)8− anions, M+ cations, and water molecules of crystallization. The Np atoms have the pentagonal bipyramidal oxygen surrounding, with five O atoms of three different octamolybdate ions arranged in the equatorial plane of the dioxocation. The complexes with the NH4, K, Rb, and Tl cations are isostructural (space group P\(\bar 1\)), whereas the complex with the Cs cation crystallizes in another space group (P21/n). The structure of the centrosymmetric octamolybdate ions (Mo8O28)8− is similar in all the compounds. The oxygen surrounding of the outer-sphere cations is characterized by a large scatter of the interatomic distances, and both the main and additional coordination numbers of the cations can be considered. The electronic and vibrational absorption spectra of the synthesized Np(V) octamolybdate complexes were measured and analyzed. The influence of the structural features of the complexes and of the kind of the outer-sphere cation on the spectral pattern is discussed.