Chemistry:Cyclopentadienyl anion

Short description: Aromatic ionic species

Cyclopentadienyl anion

Names
IUPAC name Cyclopentadienide
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

Tracking categories (test):

• PairSizeSet

In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of [C₅H₅]⁻ and abbreviated as Cp⁻. It is formed by the deprotonation of cyclopentadiene. The cyclopentadienyl anion is a ligand which binds to a metal in organometallic chemistry.

Resonance and aromaticity

Resonance structures of the cyclopentadienyl anion

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. Each double bond and lone pair provides 2 π-electrons, which are delocalized into the ring. The cyclopentadienyl anion is a conjugated system because there are alternating π and 𝜎 bonds.

Cyclopentadiene has a pKa of about 16. It is acidic relative to many carbon acids. The enhanced acidity is attributed to stabilization of the conjugate base, cyclopentadienyl anion.

Ligand

Cyclopentadienyl anions form a variety of cyclopentadienyl complexes.

Metallocene structure where "M" is a metal

See also

• Cyclopentadienyl radical, [C₅H₅]^•
• Cyclopentadienyl cation, [C₅H₅]⁺
• Cyclooctatetraenide anion, [C₈H₈]²⁻