a, Borane compounds, BR3, have a 'vacancy' — an absence of a pair of electrons (empty square) — in their shell of valence electrons. R represents any group that can donate a single atom for bond formation. b, Lewis bases, such as tertiary amines (NR3), have a 'lone pair' of electrons (filled square), which they can donate to BR3 to fill the vacancy, forming a complex. c, Borylene compounds have two vacancies and a lone pair. d, Borylene complexes can therefore form when two Lewis-base ligands (L) donate their lone pairs to the borylene. Braunschweig et al.1 report a borylene complex in which the boron atom binds two carbon monoxide ligands.