Cyclopentadienyl anion

In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of [C₅H₅]⁻ and abbreviated as Cp⁻.[1] Its name derives from the molecule cyclopentadiene.

Cyclopentadienyl anion

Identifiers
CAS Number	12127-83-2
3D model (JSmol)	Interactive image
ChemSpider	2731064
PubChem CID	3490527
InChI InChI=1S/C5H5/c1-2-4-5-3-1/h1-5H/q-1 Key: SINKOGOPEQSHQD-UHFFFAOYSA-N InChI=1/C5H5/c1-2-4-5-3-1/h1-5H/q-1 Key: SINKOGOPEQSHQD-UHFFFAOYAD
SMILES [cH-]1cccc1
Properties
Chemical formula	[C5H5]− or Cp−
Molar mass	65.09 g/mol
Conjugate acid	Cyclopentadiene
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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.

Salts of the cyclopentadienyl anion can be stable, e.g., sodium cyclopentadienide.

Coordination compounds of the cyclopentadienyl anion (not the cyclopentadienyl radical) are known as cyclopentadienyl complexes. Biscyclopentadienyl complexes are called metallocenes.