Structural Evolution and 5f Orbitals Participation in Inverse Sandwich Actinide Boride Clusters: A Theoretical Study...

Actinide-boron binary clusters have received considerable attention due to their intriguing electronic structures and chemical bonding. A series of highly symmetric inverse sandwich structures has been recently reported for the An₂B₈, so it is natural to ask whether it is possible to form such inverse sandwich clusters with larger B_n rings. Herein, we predict the possibility of the inverse sandwich actinide borides An₂B₉^- and An₂B₁₀ (An = Th, Pa, U) via first-principles theory calculations. The global minimum structures of An₂B₉^- are formed by a B₇ subunit with a perpendicular B₂ unit sandwiched by two An atoms, whereas those of An₂B₁₀ can be viewed as adding a B₂ unit off-plane to the inverse sandwich An₂B₈. Detailed molecular orbital composition analyses quantitatively demonstrate a marked increase in the participation of actinide 5f orbitals along the series Th → Pa → U in both the An₂B₉^- and An₂B₁₀ complexes. Furthermore, adaptive natural density partitioning (AdNDP) and principal interacting spin orbital (PISO) analyses on U₂B₉^- (3) reveal the U-U 5f orbital interactions and show that the U 6d-involved PISOs pairs make a dominating contribution to the coordination interaction energy of the complex, whereas the U 5f-based PISO pairs also contribute substantially to the stability of the cluster.