Crystal structure of the neptunium(VII) compound Co(NH3)6NpO4 (OH)2 x 2H2O

The authors have made a complete determination of the crystal structure of the compound Co(NH3)6NpO4(OH)2 x 2H2O (CAD 4 SDP automatic diffractometer). The crystals were of platelike form a = 0.6778(2), b = 0.8449(2), c = 1.2448(3) nm, β = 119.30(2)0, space group P21/c, Z = 2; rho calc = 2.88 g/cm3, R = 0.032, R/sub W/ = 0.040 (1423 reflections, 77 variables, refinement by full-matrix least-squares method in the anisotropic approximation for all the nonhydrogen atoms). The basic structural elements are the complex anions [NpO4(OH)2]3-, the cations [Co(NH3)63+, and the molecules of water of crystallization. The centrosymmetric complex anion has the shape of a slightly distorted tetragonal bipyramid with Np-O distances 0.1892(5) nm and 0.1884(5) nm, and Np-O(OH) distances 0.2323(5) nm. The O-Np-O angle is 90.1(3)0, while the O(OH)-Np-O angles are 91.0(2)0 and 91.2(2)0, i.e., the oxygen atom in the OH group deviates slightly from the axis of an ideal tetragonal bipyramid. All the Co-N distances in the [Co(NH3)6]3+ cation are 0.1963(5) nm. The Moessbauer data at a temperature of 4.2 γ indicates the presence of two nonequivalent Np positions