Cyclopentadienyl anion

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

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]

The first salt with this anion, potassium cyclopentadienide, was prepared by Johannes Thiele in 1901^[3] but there wasn't much interest in the topic until the discovery of ferrocene in the 1950s.

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.^[4]

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.^[5]

Metallocene structure where "M" is a metal

References

^ "Cyclopentadienide". PubChem Compound Database. National Center for Biotechnology Information. Retrieved 14 April 2016.
^ 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
^ Johannes Thiele (January 1901). "Ueber Abkömmlinge des Cyclopentadiëns". Berichte der Deutschen Chemischen Gesellschaft zu Berlin (in German). 34 (1): 68–71. doi:10.1002/CBER.19010340114. ISSN 0365-9496. Wikidata Q126217369.
^ 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.
^ C. Elschenbroich (2006). Organometallics. VCH. ISBN 978-3-527-29390-2.

[Cyclopentadienide-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] Johannes Thiele (January 1901). "Ueber Abkömmlinge des Cyclopentadiëns". Berichte der Deutschen Chemischen Gesellschaft zu Berlin (in German). 34 (1): 68–71. doi:10.1002/CBER.19010340114. ISSN 0365-9496. Wikidata Q126217369.

[4] 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.

[5] C. Elschenbroich (2006). Organometallics. VCH. ISBN 978-3-527-29390-2.

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Cyclopentadienyl anion

Contents

Resonance and aromaticity

Ligand

See also

References