**1.2 Carbene**

The term "Carbene" refers to the presence of neutral bivalent carbon with six valence electrons in N-heterocyclic compounds (**Figure 1**). The first reported carbene (I) was by Bartrand et al. in 1988 [13], as resonance stabilized ylide form. After a few years, the first stable NHC was reported by Arduengo et al. as an imidazolium ring [14]. In NHC, the singlet state of carbene is more thermodynamically favorable than triplet carbene. Because nitrogen is present near to carbon of carbene, it lowers the energy of the highest occupied molecular orbital (HOMO) while it increases the energy of the lowest unoccupied molecular orbital's. The nucleophilicity of carbene also increases (**A**) not only above energy character but also presence of inductive effect, mesomeric and lone pair to vacant p-orbital favors singlet carbene. Most NHCs are based on imidazolium, triazolium, or thiazolium ring-containing molecules. NHCs dimerize reversible in the form of the Wanzlick equilibrium (**B**) [15–16].

Since the discovery of metal carbenes in 1964 by Fisher et al. [17], fascinating applications in both catalysis and synthesis are being observed [18].
