The reaction of the neutral, base-stabilised diborenes 1a,b with the well-known hydroboration reagent 9-borabicyclo[3.3.1]nonane (9-BBN) is presented. This leads to the formation of a nonclassical structural motif with a B₃ core concomitant with the cleavage of a B–C bond. This compound has been characterised in solution and by X-ray crystallography. According to Wade's rules these rare B₃ clusters follow the arachno classification (2n + 6 electrons) and represent doubly base-stabilised B₃H₅ analogues.