The x-ray crystal structures of cryst. fluorenylidenecycloproparenes I and II and of dibenzocycloheptatrienylidene III are reported. Theor. studies, using ab initio methods at the HF/6-31G(d,p) and the correlated MP2/6-31G(d,p) levels, were used to provide assessments of the structure, charge distribution, dipole moment, and thermodn. stability of the unknown methylidenecyclopropabenzene IV, the derived parent tria-, penta-, and heptafulvalene derivs. V (n = 1-3), and the cryst. derivs. I-III. The hydrocarbons are polar, and the cycloproparenylidene moiety acts as an electron donor in all but V (n = 1), which is the only fulvalene hydrocarbon in the series calcd. to have a neg. polarized cycloproparenyl unit.