New data on Np(VII) compounds with Co(NH₃)₆³⁺. Crystal structure of [Co(NH₃)₆]₃[NpO₄(OH)₂]₃·4H₂O and refinement of the structure of [Co(NH₃)₆][NpO₄(OH)₂]·2H₂O

The structure and properties of the needle-like phase of crystalline compounds of the general composition Co(NH₃)₆NpO₅·nH₂O, precipitated from alkaline Np(VII) solutions with [Co(NH₃)₆]³⁺ ions, were studied. The needle-like phase under the layer of the mother liquor is unstable and transforms into a platelike phase identified as [Co(NH₃)₆][NpO₄(OH)₂]·2H₂O (1). In the structure of 1, all the Np atoms are equivalent, but the Mössbauer spectrum recorded at a liquid helium temperature revealed the presence of two nonequivalent Np atoms. The isolated needle-like phase, according to single crystal X-ray diffraction data, at 100 K has the composition [Co(NH₃)₆]₃[NpO₄(OH)₂]₃·4H₂O (2). In the structure of 2, there are two crystallographically independent Np(VII) atoms. The Np(1) atom is in the inversion center, and the Np(2) atom occupies a general position. The structure of 1 was additionally refined, and the structures of 1 and 2 were compared. Both compounds have a similar structure, but the structure of 2 is less ordered, and the formation of both structures is influenced by hydrogen bonds involving water molecules and [NpO₄(OH)₂]^3– ions. In the structure of 1, hydrogen bonds of type O–H···O form a three-dimensional network. In the structure of 2, the O–H···O hydrogen bonds are weaker and form a two-dimensional network. In the structure of 1, there are two water molecules per [NpO₄(OH)₂]^3– anion, and in the structure of 2, 4/3 water molecules. Needle-like phase 2 in contact with solutions transforms into stable platelike phase 1 with dense packing and a three-dimensional system of hydrogen bonds. The size of needle-like crystals increases, and the rate of their transformation into platelike crystals decreases with increasing alkali concentration and decreasing temperature. The influence of the experiment conditions on the interaction of Np(VII) with [Co(NH₃)₆]³⁺ ions in weakly alkaline (0.05–0.15 M) solutions was also studied. The precipitates formed are characterized by the ratio [NpO₄(OH)₂]^3–: [Co(NH₃)₆]³⁺ = 2: 1 and are identified as the known compound LiCo(NH₃)₆Np₂O₈(OH)₂·2H₂O.