Bullvalene C10H10 and its analogs semibullvalene C8H8, barbaralane C9H10, and 9-Borabarbaralane C8BH9are prototypical fuxional molecules with rapid Cope rearrangements at fnite temperatures. Detailed bonding analyses performed in this work reveal the existence of two fuxional π-bonds (2 2c-2e π→2 3c-2e π→2 2c-2e π) and one fuxional σ-bond (1 2c-2e σ→1 4c-2e σ→1 2c-2e σ) in their ground states and transition states, unveiling the universal π+σ double fuxional bonding nature of these fuctuating cage-like species. The highest occupied natural bond orbitals (HONBOs) turn out to be typical fuxional bonds dominating the dynamics of the systems. The 13C-NMR and 1H-NMR shielding tensors and chemical shifts of the model compound C8BH9 are computationally predicted to facilitate future experiments
