3,4-dihydro-2H-1,2,4,3-triazaborol-3-yl-lithium 3 was synthesized and fully characterized. The 11B NMR spectrum, X-ray diffraction analysis, and computational studies revealed the ionic nature of the B–Li bond, and indeed 3 displays nucleophilic property which allowed preparation of a series of 1,2,4,3-triazaborol-3-yl-metal complexes (Al; 5, Au; 6, Zn; 7, Mg; 8, Sb; 9, and Bi; 10). 3 reacted with CO (1 atm) and various isonitriles under ambient condition, and mechanistic study suggests that the reactions with CO and aryl isonitriles proceed via an insertion of CO and isonitrile carbon into the B–Li bond followed by isomerization to yield transient carbene species, one of which was confirmed by trapping with S8. With PhNC, compounds 5 and 7·(thf) underwent exchange of THF molecule coordinating to the metal center with isonitrile, whereas insertion of isonitrile carbon occurred at the B–Bi bond in 10 which afforded stable bismuth (boryl)­iminomethane 20.