The B-B bond of tri-tert-butylazadiboriridine (I) is oxidized by MeOH to give the (hydroboryl)(methoxyboryl)amine HBR:NRBR(OMe) (2a). m-2,3-Hydroazoniadiborata-1-cyclopropenes II (X = e.g., OSO2CF3, NHB10H12N3) are formed by the action of the acids HX upon I. A similar H bridge as in II is formed during the hydroboration of I by catecholborane. A B atom is identified in the hydroazoniadiborata-1-cyclopropene products to be planarly coordinated by four atoms, two of which are forming a BHB three-center bond with the B atom. This is a novel bonding situation in B chem. The structures of the products are deduced from 1H-, 11B-, and 13C-NMR spectra and are confirmed by x-ray structural analyses of 2a and II (same X as above).