NEPTUNYL COMPOUNDS: POLYHEDRON GEOMETRIES, BOND-VALENCE PARAMETERS, AND STRUCTURAL HIERARCHY

The structures of 59 inorganic Np5+ and Np6+ neptunyl compounds are examined and placed within a structural hierarchy, with special attention to the relationships of these structures with U6+ uranyl compounds. Forty-three Np5+ neptunyl compounds containing square, pentagonal and hexagonal bipyramids have structural units consisting of isolated polyhedra (2), finite clusters of polyhedra (1), chains of polyhedra (12), sheets of polyhedra (16), and frameworks of polyhedra (12). Cation-cation interactions occur in 18 of these, resulting in a dramatic departure of their topologies from those of U6+ uranyl compounds. Compounds with cation-cation interactions have structural units consisting of chains (4), sheets (4) and frameworks (10) of polyhedra. Those Np5+ compounds that lack cation-cation interactions exhibit topologies that are similar or identical to those of U6+ compounds. Sixteen Np6+ neptunyl compounds have structural units consisting of isolated polyhedra (1), finite clusters of polyhedra (3), chains of polyhedra (4), and sheets (8) of polyhedra. None of these structures exhibit cation-cation interactions, and their topologies are similar to U6+ uranyl compounds. Geometries of Np5+ and Np6+ polyhedra are examined, and updated bond-valence parameters, R-o = 2.035 and b = 0.422, are provided for Np5+.