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

Abstract

Mononuclear ternary actinide hydroxo carbonato complexes AnO2(OH)y(CO3)zq 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.

Single-Hydroxide Bridged Dimers of U and Np Actinyls: A Density Functional Study on Their Existence and Structure in Aqueous Solution

“…A potentiometry measurement determined a somewhat higher value, 23 kJ/mol . We attribute this notable deviation of our results from experiments mainly to shortcomings of the continuum solvation model when comparing species of different charges and sizes. ,− Similar deviations between experiment and calculations also have been observed for the formation reactions of other complexes modeled including the first solvation shell only. , …”

“…The uranyl cation in water prefers to coordinate five aqua ligands in its first shell, as shown in EXAFS , and NMR studies. Our earlier calculations suggest that the monohydroxo complex [UO 2 (OH)] + coordinates four aqua ligands, leading also to a total CN of 5 in the equatorial plane of uranyl. Therefore, we assume that the equatorial CN of 5 is preserved in the dimeric actinyl aggregate, as also considered previously .…”

“…For the dimer complex U­(VI)–5,5pa, we calculated average uranyl bond lengths UO of 179.4 pm, only slightly longer than the analogous terminal bonds of a U­(VI) aqua ion, 179.0 pm (Table ). These results also illustrate the weak binding due to the bridging hydroxide, as longer UO bond lengths of 181.1 pm on average have been determined for [UO 2 (OH)] + . Both uranyl units in the complex are nearly linear, with an average angle OUO of 177° (Table ).…”

“…All calculations were carried out at the generalized gradient approximation (GGA) level for the exchange-correlation functional, invoking the PBE parametrization which was shown to yield better geometries and energies than hybrid functionals for related systems. , For Np complexes, spin-unrestricted (spin-polarized) calculations were carried out. For mononuclear complexes, the scalar relativistic spin-unrestricted approach correctly yields a doublet state for Np­(VI) and a triplet state for Np­(V) species. The open-shell f orbitals are strongly localized on Np (98%) and thus are not involved in the bonding.…”

Unusually Kinetically Inert Monocationic Neptunyl Complex with a Fluorescein-Modified 1,10-Phenanthroline-2,9-dicarboxylate Ligand: Specific Separation and Detection in Gel Electrophoresis

“…The SDD, which treats the 60 core electrons (small core) as the potential to simplify the relativistic effect of heavy atoms, has often been used for transition metals , and UO 2 2+ complexes . The multiplicity ( S ) of the Np­(V)­O 2 + –PDA complex was set to 3. − Since no spin contamination was observed, ⟨ S 2 ⟩ in the ground state was equivalent to S ( S + 1), which is identical to the theoretical value. This suggested that the calculation was correctly performed.…”