Observation of Four-Fold Boron-Metal Bonds in RhB(BO) and RhB.

The maximum bond order between two main-group atoms was known to be three. However, it has been suggested recently that there is quadruple bonding in C and analogous eight-valence electron species. While the quadruple bond in C has aroused some debates, an interesting question is: are main-group elements capable of forming quadruple bonds? Here we use photoelectron spectroscopy and computational chemistry to probe the electronic structure and chemical bonding in RhBO and RhB and show that the boron atom engages in quadruple bonding with rhodium in RhB(BO) and neutral RhB. The quadruple bonds consist of two π-bonds formed between the Rh 4d/4d and B 2p/2p orbitals and two σ-bonds between the Rh 4d and B 2s/2p orbitals. To confirm the quadruple bond in RhB, we also investigate the linear Rh≡B-H species and find a triple bond between Rh and B, which has a longer bond length, lower stretching frequency, and smaller bond dissociation energy in comparison with that of the Rh≣B quadruple bond in RhB.