Due to the electron-deficient characteristics of boron, boron clusters exhibit a variety of structures with size changes, and the doping of different metal atoms can further enrich the structures and properties of boron clusters. We investigated a series of actinide metal-doped boron clusters AnB₈ (An = Ac, Th, Am, Cm) by using the global minimum structural searches and density functional theory (DFT) methods. For each isomer, different spin states were considered at the PBE0/6-311+G*/RECP and TPSSH/6-311+G*/RECP theoretical levels. It is found that each AnB₈ cluster possesses two configurations, half-sandwich and chair-like structures. Except for ThB₈, the half-sandwich structures of AcB₈, AmB₈ and CmB₈ are more stable, while the energy difference between the two structures of ThB₈ is small, indicating that both may coexist in experiments. The half-sandwich structures of AcB₈, ThB₈, AmB₈, and CmB₈ correspond to doublet, singlet, octet, and nonet states, respectively. ThB₈ is a closed-shell singlet system, in which the electronic configuration of the Th element may be [Rn]6d². VDD (Voronoi deformation density) and Hirshfeld charges suggest that charge flows from the actinide elements to the boron ligands. The half-sandwich structures of these boron clusters are stable M^II[B₈^2-]-type divalent actinide complexes. Molecular orbital (MO) analysis shows that the An-B bonding orbitals of AcB₈ and ThB₈ mainly originate from An 6d orbitals, while those of AmB₈ and CmB₈ are primarily contributed by An 5f orbitals. Other analyses of bonding properties also indicate that there exist covalent interactions between An and B atoms in all complexes, and ThB₈ has stronger covalent interactions. For these AnB₈ complexes, the B₈^2- ligands show double aromaticity features, which have six delocalized σ electrons and six delocalized π electrons. According to dissociation energy analysis, all the AnB₈ complexes are highly stable, especially ThB₈. These results indicate that the B₈^2- ligands are capable of stabilizing the divalent actinides.

Key words: boron clusters, metal doping, actinides, density functional theory, global minimum structural search