Molecules with all triple bonds: OCBBCO, N₂BBN₂, and [OBBBBO]^2-

DFT calculations at the BP86/TZ2P level have been carried out for the compounds OCBBCO, N₂BBN₂, and [OBBBBO]^2-. The calculations predict very short distances and large bond dissociation energies for the central B-B bonds. The nature of the bonding situation was investigated with an energy decomposition analysis. It shows that the central boron-boron bonds are genuine triple bonds. The π-bonding contributes between 38-40% to the total orbital interactions of the B≡B bonds. The compounds can be considered as donor-acceptor complexes L→BB←L between the central B₂ moiety in the third [(3)¹Σ_g ⁺] excited state and the ligands L = CO, N₂, BO ^-. The π-backdonation L→BB←L for L = CO, N₂ is very strong, which suggests that the latter bonds should also be considered as triple bonds. The π-bonding in [OB-BB-BO]^2- is weaker, which makes the latter bonds borderline cases for triple bonds. The triple-bond character explains the very large bond dissociation energies for the LB-BL and L-BB-L bonds.