Fulvalenes

A fulvalene is a hydrocarbon obtained by formally cross-conjugating two rings through a common exocyclic double bond.

Pentafulvalene (2) is also called simply fulvalene, the parent structure of this class.

Triapentafulvalene (3) is also known as calicene from the words calix or chalice because of its wine-glass appearance.

Several members should be stabilized taking into account a dipolar mesomeric form with for instance sesquifulvalene 4, which can be thought of as a tropylium cation joined to a cyclopentadienyl anion (both stable and aromatic).

Fulvalenes forms stable organometallic complexes that can be formally considered derivatives of the dianion C10H82−, akin to two bonded cyclopentadienyl anions.