Neptunyl Compounds: Polyhedron Geometries, Bond-Valence Parameters, and Structural Hierarchy

The structures of 59 inorganic Np 5+ and Np 6+ neptunyl compounds are examined and placed within a structural hierarchy, with special attention to the relationships of these structures with U 6+ uranyl compounds. Forty-three Np 5+ 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 U 6+ uranyl compounds. Compounds with cation–cation interactions have structural units consisting of chains (4), sheets (4) and frameworks (10) of polyhedra. Those Np 5+ compounds that lack cation–cation interactions exhibit topologies that are similar or identical to those of U 6+ compounds. Sixteen Np 6+ 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 U 6+ uranyl compounds. Geometries of Np 5+ and Np 6+ polyhedra are examined, and updated bond-valence parameters, R o = 2.035 and b = 0.422, are provided for Np 5+ .