a, Borole is an unsaturated ring system containing four carbons, one boron atom and two C–C double bonds. Borole itself is intrinsically unstable and has not been isolated. B is boron. b, Braunschweig et al.4 have isolated and characterized an anionic borole-containing complex. The complex is stabilized by the presence of an N-heterocyclic carbene (red), which donates a lone pair of electrons to the borole (arrow). Ph, phenyl group. c, The electrons in the anionic complex are delocalized, but these 'resonance structures' show ways in which the electrons could become localized between the atoms. The double-headed arrow indicates that the real structure of the complex is a hybrid of the resonance structures. The left-hand structure contains a borataalkene group (blue). In the right-hand structure, two negative charges reside on the borole ring. This means that, in this resonance structure, the borole has six π-electrons, which makes it aromatic. The existence of an aromatic resonance structure might help to stabilize the complex.