The reactivity of a bulky m-terphenylboronic acid, DmpB(OH)(2) [1; Dmp = 2,6-bis(2,4,6-trimethylphenyl)phenyll, toward three different N-heterocyclic carbenes has been examined. The reaction of 1 with 1 equiv of bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) leads to the formation of a hydrogen-bonded carbene boronic acid adduct, 2, featuring strong O-H center dot center dot center dot C contacts. In contrast, more basic 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IPr2Me2) and 1,3-di-tent-butylimidazol-2-ylidene ((IBu)-Bu-t) deprotonate 1 smoothly to afford the rare anionic boranuidacarboxylic acids 3 and 4, respectively. Structural determination reveals that 3 and 4 bear unprecedented bifurcated hydrogen bonds with a BO- unit as a double hydrogen-bond acceptor, which contribute significantly to stabilization of the highly reactive B=O double bond. Quantum-mechanical calculations were conducted to disclose the unique electronic properties of the multiple bonds, as well as the important hydrogen bonds in these compounds.