In organometallic chemistry, a metallacycle is a derivative of a carbocyclic compound wherein a metal has replaced at least one carbon center; this is to some extent similar to heterocycles. Metallacycles appear frequently as reactive intermediates in catalysis, e.g. olefin metathesis and alkyne trimerization. In organic synthesis, directed ortho metalation is widely used for the functionalization of arene rings via C-H activation. One main effect that metallic atom substitution on a cyclic carbon compound is distorting the geometry due to the large size of typical metals. Nomenclature Typically, metallacycles are cyclic compounds with two metal carbon bonds. Many compounds containing metals in rings are known, for example chelate rings. Usually, such compounds are not classified as metallacycles, but the naming conventions are not rigidly followed. Within the area of coordination chemistry and supramolecular chemistry, examples include metallacrowns, metallacryptands, metallahelices, and molecular wheels. Classes of metallacycles Metal-alkene complexes can be viewed as the smallest metallacycles, but they usually are not classified as metallacycles. In the Dewar–Chatt–Duncanson model, one resonance structure for the M(η2-alkene) center is the metallacyclopropane. Metallacyclobutanes The parent metallacyclobutane has the formula LnM(CH2)3 where L is a ligand attached to M. A stable example is (PPh3)2Pt(CH2)3. The first example was prepared by oxidative addition of cyclopropane to platinum. Metallacyclobutane intermediates are involved in the alkene metathesis and in the oligomerization and dimerization of ethylene. In alkene metathesis, the Chauvin mechanism invokes the attack of an alkene at an electrophilic metal carbene catalyst. This work helped to validate the Chauvin mechanism for olefin metathesis. Metallacyclopentadienes and metallabenzenes The parent metallacyclopentadiene, or metallole, has the formula LnM(CH)4. Most arise from the coupling of two alkynes at a low valent metal centers such as derivatives of Co(I) and Zr(II). Late metal derivatives (Co, Ni) are intermediates in the metal-catalysed trimerization of alkynes to arenes. Early metal derivatives, i.e. derivatives of Ti and Zr, are used stoichiometrically. For example, the zirconacyclopentadiene Cp2ZrC4Me4 is a useful carrier for C4Me42−. Some of the oldest metallacycles are the ferroles, which are dimetallacyclopentadiene complexes of the formula Fe2(C2R4)(CO)6. They are derived from coupling of alkynes as well as from the desulfurization of thiophenes. The parent metallacyclobenzenes have the formula LnM(CH)5. They can be viewed as derivatives of benzene wherein a CH center has been replaced by a transition metal complex. Metallacyclopentanes The parent metallacyclopentane has the formula LnM(CH2)4. Such compounds are intermediates in the metal catalysed dimerization,