Cyclopentadienyl anion

In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of [C₅H₅]⁻ and abbreviated as Cp⁻. Its name derives from the molecule cyclopentadiene. Salts of the cyclopentadienyl anion can be stable, e.g., sodium cyclopentadienide. more complex coordination compounds are known as cyclopentadienyl complexes, such as metallocene

It is a regular pentagonal, planar, cyclic ion; as well, it has 6 π-electrons (4n+2, where n=1), which fulfills Hückel's rule of aromaticity. It can coordinate as a ligand to metal atoms.

The structure shown is a composite of five resonance contributors in which each carbon atom carries part of the negative charge.