1,3,4,5-Tetra­methyl­imid­azol-2-yli­dene

C₇H₁₂N₂

38/50 uncommon

This unusual species is a member of a class of compounds called N-heterocyclic carbenes (NHC), an invaluable crop of ligands in organometallic synthesis. This particular NHC is noteworthy for its comparative lack of bulky substituents, as steric bulk is typically used to prevent carbene dimerization. A 'carbene' is a divalent carbon(2) species, which means it simultaneously possesses both nucleophilic and electrophilic character at the same carbon atom. For this reason, carbenes are occasionally represented as having a simultaneous positive and negative charge on the carbenic atom, as a useful heuristic for conceptualizing their unique biphilic reactivity. (Pedantic readers will note that this formalism is typically reserved for triplet carbenes, which this NHC is not.) NHCs are an example of "persistent" or "stable carbenes" and can in some cases even be isolated and stored – occasionally even in air. However, some theorists argue that the ground state for many NHCs is perhaps closer to a closed-shell zwitterion, and that many NHCs are not 'carbenes' at all.