The synthesis and isolation of the first stable C–B–N-substituted borinium [MesBNⁱPr₂][B(C₆F₅)₄] (2) is described. Compound 2 was shown to react with isothiocyanate and carbodiimides, effecting B–C insertion to afford nitrilium (4) and borenium amidinate salts, respectively. The borinium cation [MesBNⁱPr₂]⁺ (2⁺) was also generated from reactions of MesB(Cl)NⁱPr₂ (1) and GaCl₃, and used to prepare stable borenium amidinate salts (5–6), while the triflate borane MesB(OTf)NⁱPr₂ (7) reacts with carbodiimide to give the bis(amidinate) boronium salt [MesC(NⁱPr)₂B((NⁱPr)₂C)NⁱPr₂][OTf] (8).