Crystal structure and spectroscopic properties of double potassium neptunium(IV) sulfate

The crystal structure of double potassium neptunium(IV) sulfate was determi ned. The composition of this compound, found by X-ray diffraction analysis, is described by the formula K-5.5(H5O2)(0.5)NP(SO4)(5), H2O [CAD4 automati c diffractometer, MoK alpha radiation, graphite monochromator, theta < 32 d egrees, monoclinic cell: a = 14.053(3), b = 8.304(2), c = 18.910(2) Angstro m, beta = 107.7(1)degrees, V = 2101.6(6) Angstrom(3), Z = 4, d(cal) = 3.06 g cm(-3) space group P2(1)/c; R-1 = 0.0369 on the basis of 6045 observed re flections with F-0 > 4 sigma(F-0), wR(2) = 0.1041 on the basis of 7482 inde pendent reflections]. The compound has a mononuclear structure. The anions [NP(SO4)(5)](6-) are the main structural units of the crystal packing. Sing le-charged cations and water molecules combine anions into a three-dimensio nal skeleton. The central neptunium atom is surrounded with 10 oxygen atoms of 5 bidentate sulfate ions. The Np(IV) coordination polyhedron can hardly be described as a regular polyhedron corresponding to some kind of known i deal polyhedra. The Np-O bond lengths in the polyhedron vary within 2.380-2 .565 Angstrom (average 2.454 Angstrom). The O-Np-O bond angles are minimal, when they are formed by the oxygen atoms of the same sulfate group (56.3 d egrees-58.4 degrees, average 56.8 degrees). In the other cases, they vary w ithin fairly broad limits (64.5 degrees-83.1 degrees, average 73.9 degrees) . In the electronic absorption spectrum of crystalline K-5.5(H5O2)(0.5)NP(S O4)(5). H2O the characteristic long-wave band reflecting the coordination s urrounding of the Np central atom splits in two components.