Whereas the reduction of N-heterocyclic carbene (NHC)-stabilised cymantrenyldibromoboranes, (NHC)BBr₂Cym, in benzene results in the formation of the corresponding diborenes (NHC)₂B₂Cym₂, a change of solvent to THF yields a borylene analogue of the form (NHC)₂BCym, stabilised through a boratafulvene/borafulvenium conformation.