Theoretical insight into the unique structural stabilization mechanism in actinide-centered boride

The exploration of metal-doped boron clusters has emerged as an effective strategy for constructing boron-based three-dimensional nanomaterials, while a comprehensive understanding of the coordination chemistry and electronic structure of lanthanide (Ln) and actinides (An) is essential for elucidating their reactivity and behavior in nuclear fuel corrosion. Using a quantum-mechanical methodology at both scalar-relativistic and spin-orbit coupling levels, in conjunction with global minimization of the UB₁₆ potential energy surface, we systematically investigate the geometries and electronic structures of MB₁₆ (M = Sc, Ti, V, Co, Rh, Ir, Eu, and An = Th to Cm) species. Three distinct structural motifs emerge: C₂ drum-like structures for metals with atomic radii <130 pm (Co, Rh, Ir), a C_8v bilayer structure for intermediate radii (130 pm < radii < 140 pm, Ti), and C_5v half-cage geometries for larger radii > 140 pm (Sc, Eu, and actinides Th-Cm). Bonding analysis reveals that actinide-boron interactions are dominated by orbital contributions, contrasting with the predominantly electrostatic bonding in transition metal analogues. In particular, the strong π_An-B bonding interactions, as well as the substantial 5f participant to the π_An-B bonds, contribute to the stability of the C_5v half-cage structure of PaB₁₆ and UB₁₆, as well as their relatively strong delocalized An-B bonds. These findings establish atomic radius as the primary determinant of structural preference and demonstrate the distinctive 5f-covalent character of actinide-boron bonding, providing fundamental insights for designing actinide-containing compounds.