*3.3.1 NHCCO2 zwitterions*

The carbon dioxide adducts of NHCs can be prepared easily by passing carbon dioxide gas into a free carbene solution, and then evaporating the solvent. They are comparatively air stable and can be stored for a long time. Imidazol(in)ium-2 carboxylate derivatives act as an excellent NHC precursor for the synthesis of NHCtransition metal complexes by releasing carbon dioxide during thermolysis. The steric effect on N substituent and other electronic, steric factors affecting the stability of such precursors has been also studied [74]. Later on, imidazol(in)ium hydrogen carbonates have been shown as another excellent source of NHCs when they lose H2CO3 upon heating [75–79].
