Fulvalene

Fulvalene is an organic compound 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 more common benzenoid aromatic compoundss naphthalene and azulene. It is also known as bicyclopentadienylidene.

Fulvalene consists of two 5-membered rings, each with two double bonds, joined by yet a fifth double bond. It has D2h symmetry. It was long sought until 1958 when it was synthesized at Yale University by E. A. Matzner,  working under W. von Doering.

Perchlorofulvalene C10Cl8 is quite stable in contrast to the hydrocarbon. . Tetrathiafulvalene is an organic semiconductor.

Fulvalene as a ligand in organometallic chemistry
Fulvalene forms stable organometallic complexes that can be formally considered derivatives of the dianion C10H82-. Some such fulvalene complexes undergo reversible C-C bond scission.