The Np(V) and Pu(V) carbonate systems: thermodynamics and coordination chemistry

New stability constants for Np-V and Pu-V carbonates have been established by coupling capillary electrophoresis (CE) and ICP-MS at the ionic strength of 0.202 mol/kg(water) in NaClO4 and at various temperatures. The thermodynamic data [variation of the molar enthalpy, Delta H-r(m)(T degrees) and entropy, Delta S-r(m)(T) of reaction] associated with the formation of the AnO(2)CO(3)(-) (An = Np, Pu), AnO(2)(CO3)(2)(3-), and AnO(2)(CO4)(3)(5-) complexes were determined. A drastic decrease in molar entropy was observed between the second and third complexes, suggesting important structural changes. Ab initio molecular dynamics showed that the 1:3 complex is formed with two carbonate ligands located at a shorter distance and one at a longer distance, suggesting that the closer ligands are coordinated in a bidentate fashion to the plutonyl(V) moiety whereas the third ligand is monodentate. The variation of the electrophoretic mobilities as function of charge/size ratio confirms unambiguously that the triscarbonate complex of pentavalent plutonium structurally differs from that of hexavalent plutonium where all carbonates anions are bidentate.