A Ligand-Centered Radical Reveals Orbital-Specific Bonding Differences Between Near-Isoradial Transuranium-Actinide and Lanthanide Complexes

Understanding how metal-ligand covalency evolves across the actinide series and compared to the lanthanide series remains a central challenge in f-element chemistry because subtle changes in both the energies and radial extension of the 5f and 6d orbitals can profoundly influence chemical bonding. Here, we employ a redox-active bipyridyl ligand platform to isolate trivalent actinide and lanthanide complexes of the form Cp* 2MIII(tBu2bipy•−) (1-M; Cp* = pentamethylcyclopentadienide; tBu2bipy•− = 4,4′-di-tert-butyl-2,2′-bipyridine radical anion; M = An = Np, Pu; M = Ln = La, Ce, Pr), including the second structually-characterized examples of Np and Pu complexes with organic radicals and the first examples with the metals in the trivalent state. Comparing their structural, electronic, spectroscopic, and electrochemical properties with those of the previously reported Am congener, 1-Am, and lanthanide counterparts 1-Nd and 1-Sm suggests a consistently greater degree of metal-bipyridyl bond covalency in 1-An relative to 1-Ln, which is reflected by shorter M–N distances as well as subtle ligand-based spectroscopic and electrochemical changes. Electronic structure calculations reveal that this enhanced covalency is driven by both increased σ-covalency—primarily involving the 6d orbitals in 1-An relative to 1-Ln—as well as ligand radical-to-metal 5f π-donation, which is absent in 1-Ln. Interestingly, the interplay of these bonding interactions leads to covalency trends that do not decrease monotonically across the actinide series. Instead, computational analyses suggest that while σ-covalency decreases across 1-An, π-covalency reaches a maximum in 1-Am before then decreasing again within a hypothetical curium(III) analog, 1-Cm. This energy-matching driven enhancement of 5f π-covalency amplifies otherwise subtle differences among ions with similar sizes and coordination preferences. More broadly, these orbital-specific trends across 1-An (Np–Cm) underscore the combined roles of spatial overlap and orbital-energy matching in soft donor radicalmediated actinide bonding.