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 RhB 2 O − 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 πbonds formed between the Rh 4d xz /4d yz and B 2p x /2p y orbitals and two σ-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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.

Mobius aromaticity was developed for twisted annulenes with electron counting rules opposite to those of Huckel aromaticity. The introduction of transition metals makes it possible for planar cyclic systems to exhibit Mobius aromaticity. Here we report the first planar monocyclic metallaboron systems with Mobius aromaticity. The structures and bonding of two rheniumboride clusters are studied by high-resolution photoelectron imaging and ab initio calculations. The ReB 3 − cluster is shown to have a near-pyramidal structure, while ReB 4− is found to be a planar pentagonal ring. Chemical bonding analyses show that both ReB 4 − and ReB 4 possess four delocalized π-electrons, including two π-electrons in an orbital of Mobius topology. NICS calculations reveal strong aromatic characters in ReB 4 − and ReB 4 , consistent with the 4n electron counting rule for Mobius aromaticity.

Metallabenzenes are a class of molecules in which a CH unit in benzene is replaced by a functionalized transition-metal atom. While all-boron analogues of aromatic and antiaromatic hydrocarbons are well-known, there have not been any metallaboron analogs. We have produced and investigated two metal-doped boron clusters, ReB6 – and AlB6 –, using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra have been obtained and compared with the theoretical results. The ReB6 – cluster is found to be perfectly planar with a B-centered hexagonal structure (C 2v , 1A1), while AlB6 – is known to have a similar structure, but with a slightly out-of-plane distortion (C s , 1A′). Chemical bonding analyses show that the closed-shell ReB6 – is doubly σ- and π-aromatic, while AlB6 – is known to be σ-aromatic and π-antiaromatic. The out-of-plane distortion in AlB6 – is due to antiaromaticity, akin to the out-of-plane distortion of the prototypical antiaromatic cyclooctatetraene. The π-bonding in ReB6 – is compared with that in both benzene and rhenabenzene [(CO)4ReC5H5], and remarkable similarities are found. Hence, ReB6 – can be viewed as the first metallaboron analog of metallabenzenes and it may be viable for syntheses with suitable ligands.