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, genuinely bent actinyl structures remain rare beyond uranium. We report the first crystallographic characterization of a bent neptunyl(VI) complex, NpO₂Cl₂(phen)₂, along with a linear plutonyl species that, unexpectedly, adopts the +V oxidation state. Coordination of two 1,10-phenanthroline ligands to NpO₂²⁺ enforces pronounced bending of the Oyl–Np–Oyl unit to 162° through steric clashes with the axial phenanthroline ligand, the sharpest angle reported for any neptunyl(VI) complex. In contrast, synthesis with PuO₂²⁺ produces a reduced linear Pu(V) species, PuO₂Cl(phen)₂, underscoring the redox lability and the reluctance of plutonium to distort. Raman and IR spectra yield the first experimental stretching and interaction force constants for plutonyl(V). Comparison with values for plutonyl(VI), and for bent and linear uranyl(VI) and neptunyl(VI) compounds, shows that reduction perturbs the actinyl bond more than bending within a single oxidation state. Electrochemical and spectroscopic studies clarify phenanthroline binding to Pu(VI) and the system’s reactivity. Quantum chemical calculations indicate that bending is energetically favored in the U–Np–Pu series, but the stabilization energy decreases as the actinide atomic number increases. NBO and QTAIM analyses reveal systematic trends: increasing 5f occupancy and decreasing An–Oyl bond covalency from U to Pu. These results demonstrate that ligand-induced bending in neptunyl(VI) species is not only electronically feasible but also synthetically achievable, though plutonyl(VI) is constrained by redox reactivity. This work expands the frontier of non-linear actinyl chemistry and illuminates how structure, oxidation state, and electronic configuration interrelate across the actinyl series.