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 NaClO 4 and at various temperatures. The thermodynamic data [variation of the molar enthalpy, Δ r H m ( T °) and entropy, Δ r S m ( T ) of reaction] associated with the formation of the AnO 2 CO 3 – (An = Np, Pu), AnO 2 (CO 3 ) 2 3– , and AnO 2 (CO 4 ) 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.

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