Theoretical Investigation on One-Electron ϕ···ϕ Bonding in Diuranium Inverse Sandwich U₂B₆ Complex Enabled by a B₆ Ring

Traditional σ, π, and δ types of covalent chemical bonding have been extensively studied for nearly a century. In contrast, ϕ-type bonding involving <i>n</i>f (<i>n</i> = 4, 5) orbitals has received less attention due to their high contraction and minimal orbital overlap. Herein, we theoretically predict a singly occupied ϕ···ϕ bonding between two 5f orbitals, facilitated by B₆ group orbitals in the hexa-boron diuranium inverse sandwich structure of U₂B₆. From <i>ab initio</i> quantum chemical calculations, the global minimum structure has a septuplet state with <i>D</i>_6<i>h</i> symmetry. Chemical bonding analyses reveal that the 5f and 6d atomic orbitals of the two uranium atoms interact with the ligand orbitals of the central B₆ ring, exhibiting favorable energy matching and symmetry compatibility to form delocalized σ-, π-, δ-, and ϕ-type bonding orbitals. Notably, even though the ϕ···ϕ bonding orbital is singly occupied, it still has a significant role in stability and cannot be overlooked. Furthermore, the U₂B₆ cluster model can be viewed as a building block of UB₂ solid materials from both geometric and electronic perspectives. This work predicts the first example of ϕ···ϕ bonding, highlighting the complexity and diversity of chemical bonds formed in actinide boride clusters.