Fulvenes

Fulvenes are the class of hydrocarbon obtained by formally cross-conjugating one ring and methylidene through a common exocyclic double bond.
The name is derived from fulvene, which has one pentagonal ring. Other examples include methylenecyclopropene and heptafulvene.
Fulvenes are generally named based on the number of ring atoms. Thus methylenecyclopropene is "triafulvene", methylenecyclopentadiene is "pentafulvene", etc.

Preparation

Fulvenes are readily prepared by the condensation of cyclopentadiene and aldehydes and ketones:
Johannes Thiele is credited with discovering this reaction.
Modern synthesis of fulvenes employ buffer systems.

Properties

The cross-conjugation generally destabilizes the exocyclic double bond, as polarization of the π electrons would lead to an aromatic ring ion. Consequently, fulvenes add nucleo- and electrophiles easily. They also have a small HOMO-LUMO gap, typically leading to the eponymous visible coloration.

Ligand in organometallic chemistry

Fulvenes are common ligands and ligand precursors in organometallic chemistry. 2,3,4,5-Tetramethylfulvene, abbreviated Me₄Fv, results from the deprotonation of cationic pentamethylcyclopentadienyl complexes. Some Me₄Fv complexes are called tuck-in complexes.