The discovery two decades ago of metal-free stable carbenes, especially imidazol-2-ylidenes [Nheterocyclic carbenes (NHCs)], has led to numerous breakthroughs in organic and organometallic catalysis. More recently, a small range of complexes has been prepared in which alternative NHC isomers, namely imidazol-5-ylidenes (also termed abnormal NHCs or aNHCs, because the carbene center is no longer located between the two nitrogens), coordinate to a transition metal. Here we report the synthesis of a metal-free aNHC that is stable at room temperature, both in the solid state and in solution. Calculations show that the aNHC is more basic than its normal NHC isomer. Because the substituent at the carbon next to the carbene center is a nonbulky phenyl group, a variety of substitution patterns should be tolerated without precluding the isolation of the corresponding aNHC.For decades, carbenes, which feature a neutral divalent carbon atom with two nonbonding electrons, were considered prototypical reactive intermediates (1). Today, thanks to the availability of stable carbenes (2,3), these molecules, especially the so-called N-heterocyclic carbenes (NHCs) (I) (4-6) ( Fig. 1, top left), are recognized as versatile ligands for transition metal-based catalysts (7-10) and as metal-free organic catalysts in their own right (11)(12)(13)(14). As expected, NHCs I usually bind metals via the carbene center (C2) to give η 1 complexes II. However, in 2001, Crabtree and co-workers discovered that 2-pyridylmethylimidazolium salts react with IrH 5 (PPh 3 ) 2 to give 1 with the imidazole ring bound the "wrong way," at C5 and not at C2 (15,16) (Fig. 1, center). Since that time, a few other complexes of type IV featuring the so-called abnormal NHCs (aNHCs) (III) (17-20) as ligands have been prepared (21-23) (Fig. 1, top right). Experimental and theoretical data suggest that aNHCs III are even stronger electron-donor ligands than are NHCs I. In line with these observations, initial catalytic screening of aNHC metal complexes IV reveals promising results for the activation of unreactive bonds such as C-H and H-H (24-26). As an example, an aNHC palladium complex has been reported to be an efficient catalyst in the Heck olefination of aryl bromides, whereas the corresponding NHC analog is virtually inactive under identical conditions (24).* To whom correspondence should be addressed. guy.bertrand@ucr.edu. Supporting Online Material NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptLassaletta and co-workers (27) have shown that the deprotonation of imidazo [1,5-a] pyridinium salts 2 leads to free NHC 3 that can be isolated ( Fig. 1, bottom). In contrast, using C2-substituted precursors, such as 4, Lassaletta et al. did not observe the corresponding free aNHC. However, by performing the deprotonation reaction in the presence of [Rh(COD)Cl] 2 , they were able to isolate the corresponding aNHC complex 5. Because calculations predicted that the parent aNHC III (where R is equal to H) is only about 17 kcal mol −1 ...