Resonance structures, fulvalenes

The fulvalene systems are not predicted to be aromatic by any of the theoretical estimates of stability. Even simple resonance considerations would suggest polyene behavior, since only dipolar resonance structures can be drawn in addition to the single nonpolar structure. 

Calicene" (1), named by Prinzbach in 1964 after the Latin calix (cup)/ is the common name for the cyclopropenylidenecyclopentadiene system and a kind of extended triafulvene as well as a member of the fulvalenes. Like triafulvene (2) and sesquifulvalene (3), calicene (1) should be resonance stabilized by the contribution of the dipolar structure IB in which the two rings assume an aromatic structure. 

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