Fulvenes
A fulvene is a hydrocarbon obtained by formally cross-conjugating one ring and Methylidene through a common exocyclic double bond.[clarification needed]
The name is derived from the similarly structured fulvenes, which lack one ring.[contradictory] Pentafulvene (2) is also called simply fulvene, the parent structure of this class.
Fulvenes are isomer of annulene