A new strategy to construct electron-precise B-B single bond via direct borylene insertion into B-H bond is described. Reduction of bromoborylene stabilized by carborane-fused silylenes with 2 equiv of sodium gives a new compound with a B-B single bond. Both experimental and density functional theory results suggest that such an electron-precise B-B single bond is formed via in situ generated borylene insertion into B-H bond.