Cyclopentadienyl anion

In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of [C
₅H
₅]⁻
and abbreviated as Cp⁻.^[1] It is formed by the deprotonation of cyclopentadiene. The cyclopentadienyl anion is a ligand which binds to a metal in organometallic chemistry.^[2]

Cyclopentadienyl anion

Names
IUPAC name Cyclopentadienide
Identifiers
CAS Number	12127-83-2
3D model (JSmol)	Interactive image
ChemSpider	2731064
PubChem CID	3490527
CompTox Dashboard (EPA)	DTXSID601027108
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

Resonance and aromaticity

See also: Sodium cyclopentadienide

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.^[3]

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

Main article: Cyclopentadienyl complex

Cyclopentadienyl anions form a variety of cyclopentadienyl complexes.^[4]

Metallocene structure where "M" is a metal

See also

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

References

1. "Cyclopentadienide". PubChem Compound Database. National Center for Biotechnology Information. Retrieved 14 April 2016.
2. Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 66, ISBN 978-0-471-72091-1
3. Paul, Satadal; Goswami, Tamal; Misra, Anirban (2015-10-01). "Noncomparative scaling of aromaticity through electron itinerancy". AIP Advances. 5 (10): 107211. doi:10.1063/1.4933191.
4. C. Elschenbroich (2006). Organometallics. VCH. ISBN 978-3-527-29390-2.