The reactions of carbodiimides with ironborylene and bis(borylene) complexes yield a variety of products, including known FeBNC and novel FeBNB metallacycles, complexes of N-heterocyclic boracarbene ligands, and a unique 1,3,2,4-diazadiborolyl pianostool complex. The product distributions can be rationalized by considering sequences of cycloaddition, metathesis, insertion, and C−H activation pathways