Bicalicene
Bicalicene is polycyclic hydrocarbon with chemical formula CH, composed of two cyclopentadiene and two cyclopropene rings linked into a larger eight-membered ring. There are two isomers: "cis"-bicalicene and "trans"-bicalicene. It is a dimer of calicene. Synthesis. Bicalicene is prepared by treatment of 1,2-bis("tert"-butylthio)-3,3-dichlorocyclopropene with cyclopentadiene anion, followed by desulfurizing stannylation with tributyltin hydride, and then treatment with silica gel. Properties. "trans"-Bicalicene is polycyclic aromatic hydrocarbon, which is unusual for a 16 π electron ring system. Viewed as a unified ring structure, Hückel's rule predicts it would be anti-aromatic (4"n" π electrons). Instead, however, the  structure has a dominant partially-delocalized charge-separated structure consisting of four independently-aromatic (4"n"+2 π electron) rings: two as cyclopropenyl cations (two π electrons each) and two as cyclopentadienyl anions (six π electrons each). "cis"-Bicalicene, by contrast, is an antiaromatic hydrocarbon. A resonance structure with four aromatic rings, analogous to the one that makes the "trans" isomer stable, would suffer from destabilizing charge effects, and other resonance structures have 4"n" rather than 4"n"+2 π electrons in at least one ring.