In this study, we compare the dimerization of N,N'-diamidocarbene with that of N-heterocyclic carbene (NHC). Less interaction occurred between the filled lone pair of nitrogen and the unfilled lone pair of the carbenic center for a N,N'-diamdiocarbene than did in a saturated NHC because of the resonance between the lone pair of nitrogen and a carbonyl group. Therefore, a N,N'-diamidocarbene exhibits less singlet-triplet splitting. The less singlet-triplet splitting in a heterocyclic carbene containing nitrogen, the more exothermic the dimerization, which is consistent with the conclusion of Thiel et al. (Chem Phys Lett 217:11-16, 1994).