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.

Cyclopentadienyl anion
Identifiers
3D model (JSmol)
ChemSpider
  • 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
  • [cH-]1cccc1
Properties
[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 delocalizes into the ring.[2] The cyclopentadienyl anion is a conjugated system because there are alternating π and 𝜎 bonds.[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 Characteristics
Main article: Cyclopentadienyl complex
Metallocene structure where "M" is a metal

The cyclopentadienyl anion is a common ligand.[4] Cyclopentadienyl complexes are often prepared by salt metathesis reactions using lithium and sodium cyclopentadienide salt. The cyclopentadienyl anion can be viewed as a polydentate ligand when it binds in the [[hapticity|η5] bonding mode.[5][6] The carbon with the lone pair and negative formal charge is an X-type ligand (anionic electron donor) and both double bonds are L-type (neutral electron donors). Since the cyclopentadienyl anion has two L-type ligands and one X-type ligand, it is an L2X ligand.[5][6] The cyclopentadienyl anion has 5 atoms in an uninterrupted π system that bind to a metal, so it has a maximum hapticity of 5.

See also

References
  1. "Cyclopentadienide". PubChem Compound Database. National Center for Biotechnology Information. Retrieved 14 April 2016.
  2. 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.
  3. "Delocalised electrons- Definition and Examples of Delocalized electrons with FAQs". BYJUS. Retrieved 2023-04-01.
  4. "Cyclopentadienyl Anion - Introduction, Cyclopentadienyl Anion Complexes, Synthesis and Applications of Cyclopentadienyl Anion". BYJUS. Retrieved 2023-04-01.
  5. "13.3.2: Simplifying the Organometallic Complex by Deconstruction". Chemistry LibreTexts. 2022-06-14. Retrieved 2023-04-01.
  6. "Simplifying the Organometallic Complex (Part 3)". Chemistry LibreTexts. 2013-10-02. Retrieved 2023-04-14.
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