Crystallization of salts M3[NpO4(OH)2]·nH2O (M = Na, K, Rb, Cs) from concentrated alkali solutions at low temperatures. Synthesis and structure of a new Np(VII) compound, Na3[NpO4(OH)2]·6H2O

Abstract

Precipitation of salts M3[NpO4(OH)2]·nH2O (M = Na, K, Rb, Cs) from concentrated alkali solutions at low temperatures (about −10°C) was studied. From solutions with [OH−] > 9.5 M, these compounds are isolated as coarse black crystals in high yield without impurity of other phases. The K, Rb, and Cs salts crystallize in the form of the previously studied compounds K3[NpO4(OH)2]·2H2O and M3[NpO4(OH)2]·3H2O (M = Rb, Cs). In the case of Na, a new hydrate Na3[NpO4(OH)2]·6H2O was obtained, and its crystal structure was determined. Crystals of the hexahydrate consist of centrosymmetrical tetragonal-bipyramidal anions [NpO4(OH)2]3−, crystallographically independent Na(1) and Na(2) cations, and water molecules. The coordination surrounding of the Np atom is characterized by noticeable difference (Δ = 0.0203 A) in the Np-O bond lengths in the equatorial plane of the bipyramid. The [NpO4(OH)2]3− anions are combined with the Na(2) cations to form infinite chains [Na(2)NpO4(OH)2(H2O)2]2− in such a manner that the lateral edges of the anion are simultaneously the lateral edges of the Na(2) coordination polyhedron. Incorporation of one of the two crystallographically independent O atoms of the NpO4 group into the Na(2) coordination surrounding is responsible for a noticeable difference in the Np-O bond lengths in the equator of the [NpO4(OH)2]3− anion. The types of hydrogen bonding in the structures of Na3[NpO4(OH)2]·nH2O (n = 0, 2, 4, 6) are compared.