Mononuclear Hydroxo Carbonato Complexes of Np(V), Np(VI), and U(VI): A Density Functional Study

Mononuclear ternary actinide hydroxo carbonato complexes AnO₂(OH)_y(CO₃)_z^q have been studied computationally for An = Np(V), Np(VI), and U(VI). Density functional calculations were carried out to examine complexes with up to two carbonate and two hydroxide ligands. This study focusses on ternary Np(V) complexes suggested by experiment. Geometry parameters of hydroxo carbonato complexes lie between the corresponding values of hydroxo and carbonato complexes with the same number of anionic ligands. We used calculated bond lengths to interpret an EXAFS experiment. We determined a tendency to decreasing complexation energies for series of complexes with a constant number of anionic ligands and increasing numbers of carbonate ligands. Our results suggest that mononuclear hydroxo carbonato complexes should also exist for An(VI). Complexation energy trends of An-OH-CO₃ complexes also yield a new perspective on the open debate on complexation constants of Np(V) hydroxo complexes. The consistently calculated properties of the An-OH-CO₃ system and their trends will be helpful for further experimental characterization.