Density functional Tao-Perdew-Staroverov-Scuseria calculations with all-electron correlation-consistent polarized valence double-xi basis set demonstrate that N-heterocyclic carbene (NHC) nickel complexes catalyze the dehydrogenation of ammonia-borane, a candidate for chemical hydrogen storage, through proton transfer from nitrogen to the metal-bound carbene carbon, instead of the B-H or N-H bond activation. This new C-H bond is then activated by the metal, transferring the H to the metal, then forming the H-2 by transferring a H from B to the metal, instead the P-H transfer. This reaction pathway explains the importance of the NHC ligands in the dehydrogenalion and points the way to finding new catalyst with higher efficiency, as partial unsaturation of the M-L bond may be essential for rapid H transfers.