Abstract
Traditional σ, π, and δ types of covalent chemical bonding have been extensively studied for nearly a century. In contrast, ϕ-type bonding involving nf (n = 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 B(6) group orbitals in the hexa-boron diuranium inverse sandwich structure of U(2)B(6). From ab initio quantum chemical calculations, the global minimum structure has a septuplet state with D(6h) 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 B(6) 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 U(2)B(6) cluster model can be viewed as a building block of UB(2) 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.