Reactions of Np(VI) and Pu(VI) with Phenanthroline Result in Bending the Np(VI)O₂²⁺ Unit and a Reduced Pu(V) Species

Despite decades of actinyl chemistry, structures containing truly bent actinyls remain rare and experimentally elusive beyond uranium. Here, we report the first crystallographic characterization of a bent neptunyl(VI) complex, NpO₂Cl₂(phen)₂, and a similar linear plutonyl species which, unexpectedly, adopts the +V oxidation state. Coordination of two 1,10-phenanthroline ligands to NpO₂²⁺ enforces significant bending of the Oyl–Np–Oyl unit to 162° through unfavorable steric interactions of the axial phenanthroline ligand with the oxos of neptunyl, representing the most acute angle observed for any neptunyl(VI) complex to date. In contrast, similar synthetic conditions with PuO₂²⁺ consistently yield a reduced, linear Pu(V) species, PuO₂Cl(phen)₂, underscoring plutonium’s redox lability and resistance to geometric distortion. For the first time, the stretching and interaction force constants of plutonyl(V) are determined from experimentally measured Raman and IR spectra. Comparison to the constants for plutonyl(VI) and bent and linear uranyl(VI) and neptunyl(VI) compounds indicates that reduction causes a larger perturbation to the actinyl bond than bending in a given oxidation state. Electrochemical and spectroscopic experiments give insight into the binding of phen to Pu(VI) and the reactivity of the system. Quantum chemical calculations confirm that bending is energetically favored across the U–Np–Pu series, but that the stabilization energy decreases with increasing actinide atomic number. NBO and QTAIM analyses reveal systematic trends in actinide–ligand bonding, including increasing 5f orbital occupancy and decreasing An–O bond covalency from U to Pu. These findings demonstrate that ligand-induced bending in neptunyl(VI) species is not only electronically feasible but also synthetically achievable—though redox reactivity may impose practical limits on bending the plutonyl(VI) moiety. Our work expands the frontier of non-linear actinyl chemistry and highlights the nuanced interplay between structure, oxidation state, and electronic configuration across the actinyl series.