Fulvenes

Fulvenes are the class of hydrocarbon obtained by formally cross-conjugating one ring and methylidene through a common exocyclic double bond.

Fulvenes are generally named based on the number of ring atoms.

[3] Fulvenes are readily prepared by the condensation of cyclopentadiene and aldehydes and ketones: Johannes Thiele is credited with discovering this reaction.

[6][7] The cross-conjugation generally destabilizes the exocyclic double bond, as (per Hückel's rules) polarization of the π electrons would lead to an aromatic ring ion.

They also have a small HOMO-LUMO gap, typically leading to the eponymous visible coloration ("fulvus" is Latin for "yellow").