A carbene is a molecule that contains a carbon atom with two bonds and two unpaired valence electrons. This leaves the carbon atom neutral in terms of formal charge (see “Atoms and Molecules”), but still typically much more reactive than a typical carbon atom with four bonds. Carbenes are often found coordinated to metal centers in organometallic complexes. These carbene ligands are less reactive than a free carbene species, and actually you might be surprised to learn that organometallic carbene complexes aren’t always prepared from the reactions of free carbenes with metal centers. As an organometallic ligand, a carbene may be be either electrophilic or nucleophilic at the carbon atom (see “Organic Chemistry”). Carbene ligands that are electrophilic at carbon are termed Fischer carbenes, and those that are nucleophilic at carbon are called Schrock carbenes. A third class of particularly unreactive carbene ligands are termed persistent (or Arduengo) carbenes.