Unlocking Novel δ and φ Bonding Modes in Actinides via Oxidation State Control.

Understanding the principles that govern actinide-ligand (An-L) bonding is essential for advancing practical applications in nuclear industry and environmental protection, as well as for deepening our fundamental knowledge of actinide chemistry. Modifying the symmetry or softness of the coordinating ligand, or altering the metal center, are common strategies to modulate the energy and orbital overlap in An-L interactions, driving both experimental and computational research efforts toward greater control over covalency. On the metal side, reducing the oxidation state causes the f- and d-orbitals to become more diffuse and destabilized. This not only enhances covalency when coordinated to suitable ligands but also opens the door to novel bonding modes via metal-to-ligand back-donation which despite their potential for advancing separation chemistry, remain largely underexplored. On the ligand side, symmetry plays a critical role in controlling the types of bonding modes. In this work, we demonstrate that variations in actinide oxidation state across the early actinide series can be used as a lever to selectively activate or suppress back-bonds. By selecting three ligands-allyl, cyclocumulene, and cyclopropene-each possessing symmetries conducive to δ and φ back-bond formation, we identified a previously elusive φ "head-to-head" back-bond. This interaction emerged as the strongest in uranium and protactinium diallyl complexes, surpassing the φ back-bonds observed in cyclooctatetraene (COT) systems. Additionally, an extension of the Dewar-Chatt-Duncanson model to f-elements is proposed. These findings not only advance our fundamental understanding of actinide bonding but also open new pathways for 5f-electrons-driven chemistries.