The X-ray crystallographic structures of crystalline fluorenylidenecycloproparenes 7 and 8 and of dibenzocycloheptatrienylidene 9 are reported. Theoretical studies, using ab initio methods at the HF/6-31G(d,p) and the correlated MP2/6-31G(d,p)) levels, have been used to provide assessments of the structure, charge distribution, dipole moment, and thermodynamic stability of the unknown methylidenecyclopropabenzene 3, the derived parent tria-, penta-, and heptafulvalene derivatives 4-6, and the crystalline derivatives 7, 8, and 9. The hydrocarbons are found to be polar, End the cycloproparenylidene moiety acts as electron donor in all but the cyclopropenylidene 4; this compound is the only fulvalene hydrocarbon in the series calculated to have a negatively polarized cyclopropareneyl unit.