Aromatic ionic species
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.
The first salt with this anion, potassium cyclopentadienide, was prepared by Johannes Thiele in 1901 but there wasn't much interest in the topic until the discovery of ferrocene in the 1950s.
Resonance and aromaticity.
The cyclopentadienyl anion is a planar, cyclic, regular-pentagonal ion; it has 6 π-electrons (4"n" + 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. 
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.
References.
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