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-2 and analogous eight-valence electron species. While the quadruple bond in C-2 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 pi-bonds formed between the Rh 4d(xz)/4d(yz) and B 2p(x)/2p(y) orbitals and two sigma-bonds between the Rh 4d(z)(2) and B 2s/2p(z) orbitals. To confirm the quadruple bond in RhB, we also investigate the linear Rh equivalent to 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(sic)B quadruple bond in RhB.