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, N(2)BBN(2), 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 pi-bonding contributes between 38-40% to the total orbital interactions of the B[triple bond]B bonds. The compounds can be considered as donor-acceptor complexes L-->BB<--L between the central B(2) moiety in the third [(3)(1)Sigma(g)(+)] excited state and the ligands L = CO, N(2), BO(-). The pi-backdonation L<--BB-->L for L = CO, N(2) is very strong, which suggests that the latter bonds should also be considered as triple bonds. The pi-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.