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 C₂ and analogous eight-valence electron species. While the quadruple bond in C₂ 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 RhB₂O^- 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.