Kim, Synthesis, Characterization, and Spectroscopic Analysis of Antiaromatic
Benzofused Metalloporphyrins, Chem.
They're classified as antiaromatic
molecules, which chemical theory predicts will be extremely reactive--in contrast to aromatics, such as benzene, which have unusually stable structures.
The cyclopentadienyl cation is a common textbook example of an antiaromatic
molecule, a molecule so electronically unstable and, therefore, extremely reactive that it should not exist for any length of time.