Hydrogen-Induced, Unexpectedly Strong 8c-2e ϕ Bond in An₂H₈ (An = Pa, Pu) Complexes

In the hierarchy of molecular orbital interactions, metal-metal ϕ bonding constitutes the zenith of angular momentum components, yet its realization in compounds remains perhaps the most striking manifestation of the stringent spatial demands required for effective orbital overlap. In this study, we report the theoretical identification of an unprecedented eight-center two-electron (8c-2e) ϕ bond, which is induced by six hydrogen atoms within the U-H₆-U inverse sandwich subunit of the U₂H₈ diactinide hydride complex with a H-U-H₆-U-H geometric configuration. This finding is supported by combined DFT and high-level ab initio calculations based on the CASPT2 method, which yield mutually consistent results. The U-H₆-U inverse sandwich unit features strong 5f_ϕ-1s_σ interactions between the two uranium atoms and the central bridging H₆ moiety. Further bonding analysis reveals the delocalized multicenter σ, π, δ, and ϕ chemical bonds, with AIMD simulations confirming the thermal stability of U₂H₈. Extending the investigation across the actinide series (An = Th-Am), DFT calculations reveal that Pa₂H₈, Np₂H₈, and Pu₂H₈ adopt a similar H-An-H₆-An-H geometric configuration featuring an 8c-2e ϕ bond. These findings enhance the fundamental understanding of covalency origins in f-block element hydride compounds, especially for uranium-based systems.