A comparison of neptunyl(V) and neptunyl(VI) solution coordination: The stability of cation-cation interactions

The solution coordination environments of pentavalent and hexavalent Np are studied by high-energy X-ray scattering. Np5+ and Np6+ both exist as the neptunyl moiety coordinated with five equatorial waters at Np-0 distances of 2.46(2) and 2.37(2) A, respectively. NPO22+ also has a second coordination sphere of 6-10 waters at 4.37(3) angstrom. The NpO2+ scattering is complicated by the presence of scattering at about 4.2 angstrom that is attributed to Np-Np cation-cation interactions. The analysis of changing intensity of this peak as a function of Np concentration is used to determine a stability constant of K-eq 0.74(9) M-1 for the dimeric complex.