Triafulvalene
Triafulvalene or cyclopropenylidenecyclopropene is a fulvalene hydrocarbon with chemical formula C6H4, composed of two linked cyclopropene rings. Triafulvalene has never been isolated, since it can decompose via an isodesmic reaction. However, its structure, stability, and spectral properties are well-studied theoretically; the molecule is believed planar.