In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of [C5H5]− and abbreviated as Cp−.

The cyclopentadienyl anion is a ligand which binds to a metal in organometallic chemistry.

[2] The first salt with this anion, potassium cyclopentadienide, was prepared by Johannes Thiele in 1901[3] but there wasn't much interest in the topic until the discovery of ferrocene in the 1950s.

The cyclopentadienyl anion is a planar, cyclic, regular-pentagonal ion; it has 6 π-electrons (4n + 2, where n = 1), which fulfills Hückel's rule of aromaticity.

The enhanced acidity is attributed to stabilization of the conjugate base, cyclopentadienyl anion.