High-level DFT calculations, coupled with appropriate isodesmic reactions, are employed to investigate the effects of monoheteroatom substitution, cyclization, and unsaturation on the stability, multiplicity, and reactivity of amino-, oxy-, silyl-, phosphino-, and thioalkylcarbenes. The results of these calculations are compared to those of di-tert-butylcarbene, 2,2,5,5- tetramethylcyclopentanylidene, and 2,2,5,5-tetramethylcyclopent-3-enylidene as the reference molecules. The calculated singlet?triplet energy gaps (ΔES?T) demonstrate the following trend: (amino ≈ oxy) > thio > phosphino > alkyl > silyl. In contrast to the previous reports, isodesmic reactions show that ?-donor/?-acceptor amino substituents stabilize not only the singlet but also the triplet states. The stabilization of the triplet states by amino substitution is much less than the singlet states. The ΔES?T values of all the carbenes are increased through cyclization, while the introduction of unsaturation causes small and rather random changes. These changes are carefully probed by means of isodesmic reactions for the singlet and triplet states, separately. The reactivity of the species is discussed in terms of nucleophilicity, electrophilicity, and proton affinity issues showing amino- and phosphinoalkylcarbenes to be more nucleophilic, more basic, and less electrophilic than oxy- and thioalkylcarbenes, respectively. This detailed study offers new insights into the chemistry of these novel carbenes.