Cation–Cation Interaction in Np(V) Perchlorates and Their Electronic Absorption Spectra

Four new Np(V) perchlorate complexes of the composition [NpO₂(H₂O)₄]ClO₄ (orthorhombic) (1), [(NpO₂)₂(ClO₄)(H₂O)₄]ClO₄·H₂O (3), [(NpO₂)(UO₂)(H₂O)₈](ClO₄)₃ (4), and [(NpO₂)₂Cl(H₂O)₄]ClO₄·2H₂O (5) have been synthesized and structurally characterized. The structure of previously known monoclinic modification of NpO₂ClO₄·4H₂O (2) has been determined at 100 K. The coordination environment of Np and U atoms in compounds 1–5 is pentagonal bipyramids. A cation–cation (CC) interaction was found in structures 1–5, in which the NpO₂⁺ ion manifests itself as a monodentate (1, 2) or bidentate (3–5) ligand. CC bonds play a fundamental role in the crystal structures of compounds 1, 2, and 4. They are the main structure-forming bonds that form the crystal package as a whole. The spectra of all compounds in the visible-near-IR range were measured.