A fulvalene is a

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...

obtained by formally cross-conjugating two

rings Ring may refer to: * Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry * To make a sound with a bell, and the sound made by a bell :(hence) to initiate a telephone connection Arts, entertainment and media Film and ...

through a common

exocyclic In organic chemistry, an alicyclic compound contains one or more all-carbon rings which may be either saturated or unsaturated, but do not have aromatic character. Alicyclic compounds may have one or more aliphatic side chains attached. The ...

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...

. The name is derived from the similarly structured fulvenes which lack one ring. Pentafulvalene (2) is also called simply

fulvalene Fulvalene (bicyclopentadienylidene) is the member of the fulvalene family with the molecular formula C10H8. It is of theoretical interest as one of the simplest non-benzenoid conjugated hydrocarbons. Fulvalene is an unstable isomer of the mor ...

, the parent structure of this class. Triapentafulvalene (3) is also known as

calicene Calicene or triapentafulvalene is a hydrocarbon of the fulvalene class with chemical formula C8H6, composed of a cyclopentadiene ring and a cyclopropene ring linked by a double bond. Its name is derived from the Latin ''calix'' meaning "goblet", ...

from the words ''calix'' or ''

chalice A chalice (from Latin 'mug', borrowed from Ancient Greek () 'cup') or goblet is a footed cup intended to hold a drink. In religious practice, a chalice is often used for drinking during a ceremony or may carry a certain symbolic meaning. Re ...

'' because of its wine-glass appearance. In general, the parent fulvalenes are very unstable; for instance, the parent

triafulvalene Triafulvalene or cyclopropenylidenecyclopropene is a fulvalene hydrocarbon with chemical formula C6H4, composed of two linked cyclopropene rings. Triafulvalene has never been isolated, since it can decompose via an isodesmic reaction. However, this ...

(1) has never been synthesized. On the other hand, stable fulvalenes can be obtained by proper substitution or

benzannulation In organic chemistry annulation (from the Latin ''anellus'' for "little ring"; occasionally annelation) is a chemical reaction in which a new ring is constructed on a molecule. : Examples are the Robinson annulation, Danheiser annulation and cert ...

. Several members should be stabilized taking into account a dipolar mesomeric form with for instance

sesquifulvalene Sesquifulvalene or Pentaheptafulvalene is a hydrocarbon in the fulvalene class with chemical formula C12H10. It is composed of linked cyclopentadiene and cycloheptatriene rings. Properties In the ground state, which is a singlet state, the cent ...

4, which can be thought of as a

tropylium cation In organic chemistry, the tropylium ion or cycloheptatrienyl cation is an aromatic species with a formula of 7H7sup>+. Its name derives from the molecule tropine from which cycloheptatriene (tropylidene) was first synthesized in 1881. Salts of ...

joined to a

cyclopentadienyl Cyclopentadienyl can refer to *Cyclopentadienyl anion, or cyclopentadienide, **Cyclopentadienyl ligand *Cyclopentadienyl radical, • *Cyclopentadienyl cation, See also *Pentadienyl In organic chemistry, pentadienyl refers to the organic radic ...

anion (both stable and

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

). In this compound the dipolar structure is calculated to contribute 23% to the total structure.

Fulvalenes as a ligand

: Tungsten fulvalene complex Fulvalenes forms stable

organometallic Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

complexes that can be formally considered derivatives of the dianion C₁₀H₈²⁻, akin to two bonded

cyclopentadienyl anion In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of and abbreviated as Cp−. It is formed from the deprotonation of the molecule cyclopentadiene. Properties The cyclopentadienyl anion i ...

Ferrocene Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, a ...

was isolated from an attempted synthesis of pentafulvalene. Many compounds are known, especially for the early transition metals. The bond joining the two rings in some fulvalene complexes can break reversibly.Boese, R. J.; Cammack, K.; Matzger, A. J.; Pflug, K.; Tolman,W. B.; Vollhardt, K. P. C.; Weidman, T. W. "Photochemistry of (Fulvalene)tetracarbonyldiruthenium and Its Derivatives: Efficient Light Energy Storage Devices" Journal of the American Chemistry Society 1997, volume 119, p. 6757-6773.

References

{{Reflist Fulvalenes