Also well studied are the mesoionic carbenes based on 1,2,3-triazole, referred to as 1,2,3-triazol-4(or 5)-ylidenes.  The first triazolylidenes were reported by Albrecht in 2008.  These carbenes are typically trisubstituted with alkyl groups in the N1 and N3 positions and an aryl group in the C4 or C5 position.  Free carbenes as well as numerous transition metal complexes have been reported.  Free carbenes that are alkylated at N3 tend to undergo decomposition reactions in which the alkyl group participates in a nucleophilic attack at the carbene position.  If N3 is substituted with a bulky alkyl group or an aryl group, the stability of the carbene increases significantly.