Carbenen

Carbenen, in chemistry often referred to as carbenes in English, are a class of reactive species characterized by a carbon atom that is divalent and neutral, bearing only six valence electrons. The carbon carries two nonbonded electrons, which can reside in a single orbital (singlet carbene) or as two unpaired electrons in separate orbitals (triplet carbene). This electronic structure makes carbenes highly reactive and, in many cases, short-lived, unless stabilized by substituents or metal coordination.

In terms of state and origin, carbenen can be categorized as free carbenes or metal-associated carbenes. Free carbenes exhibit distinct spin states: singlet versus triplet, which strongly influence their reactivity. Singlet carbenen tend to participate in two-electron processes such as cyclopropanation with alkenes or insertion into C–H bonds, while triplet carbenen show radical-like reactivity, including addition to multiple bonds and homolytic cleavages.

A prominent subdivision arises in organometallic chemistry with Fischer- and Schrock-type carbenen. Fischer carbenen are associated with late transition metals and generally feature an electrophilic carbon center stabilized by electrondonating ligands; they are typically singlet. Schrock carbenen stem from early transition metals and possess a more nucleophilic carbene carbon, often existing in a triplet state. A separate but widely important family comprises N-heterocyclic carbenen (NHCs), stable carbenen embedded in heterocycles, such as imidazol-2-ylidenes, used extensively as ligands in homogeneous catalysis.

Applications include catalytic cyclopropanation and C–H or O–H/N–H insertions in synthesis, as well as deployment as ligands in metathesis, hydrogenation, and cross-coupling reactions. Carbenen are typically generated in situ under controlled conditions due to their high reactivity and sensitivity.