Reaction of the (B,C-eta(2))-1-aza-2-borabutatriene rhodium complex 1 with 1,3-dimethylimidazol-2-ylidene) (IMe, 2) afforded the N-heterocyclic carbene-stabilized (C,C-eta(2))-1-boraallene rhodium complex 3, which has been characterized in solution and by X-ray crystallography. Density functional theory calculations were carried out to elucidate the observed base-induced B-C to C-C coordination mode shift, which suggested that the latter is 25 kJ/mol lower in energy.