Cyclopentadienyl anion

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	[C 5H 5]− 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

In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of [C
₅H
₅]⁻
and abbreviated as Cp⁻. It is formed from the deprotonation of the molecule cyclopentadiene.

Properties

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 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. It can also coordinate as a ligand to metal atoms, forming coordination compounds known as cyclopentadienyl complexes. Biscyclopentadienyl complexes are called metallocenes.

Cyclopentadienyl, [C
₅H
₅]⁻
, and cyclopentadiene, C
₅H
₆, can substitute one or more hydrogens, forming derivatives having covalent bonds. (See Cyclopentadiene#Derivatives)

Abbreviation

The abbreviation Cp for cyclopentadienyl played a role in the naming of element 112, copernicium, for which the originally proposed symbol was Cp. However, as Cp⁻ was already in use as the abbreviation for cyclopentadienyl (and Cp had been used for cassiopeium, a former name of lutetium), the symbol for copernicium was changed to Cn.

See also

• Cyclopentadienyl radical, [C
  ₅H
  ₅]^•
• Cyclopentadienyl cation, [C
  ₅H
  ₅]⁺
  
• Cyclooctatetraenide anion, [C
  ₈H
  ₈]²⁻