Title: Theoretical studies on the oxidation states and electronic structures of actinide-borides: AnB₁₂ (An = Th–Cm) clusters

Abstract: As B₁₂ clusters exhibit significant structural stability due to double aromaticity, metal doped-B₁₂ clusters often prefer a half sandwich structure. Herein, we report a systematic theoretical study on the geometric and electronic structures, and chemical bonding of the half sandwich AnB₁₂ (An = Th to Cm) clusters to explore the stability and extent of covalency of the An–B bonds of these actinide borides. We have shown that in the gas-phase clusters, the significant stability of AnB₁₂ is determined by electrostatic and orbital interactions between the An 5f6d7s orbitals and π-type molecular orbitals from B 2p orbitals of the B₁₂ unit. A change-over of An–B bond length from An = Th to Cm is found at An = Pa as a result of actinide contraction combined with weakening An–B bonding due to an energy decrease and orbital localization of the 5f orbitals. Consistently, the oxidation states of the An atoms at first increase from Th(f⁰)^IV to Pa(f⁰)^V, and then due to the 5f-AO contraction, they smoothly decline to U(f²)^IV, Np(f⁴)^III and Pu(f⁵)^III, and then eventually to Am(f⁷)^II but Cm(f⁷)^III, both with a half-filled 5f shell.