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Concept
Cyclopentadienyl anion
Summary
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.
Sodium cyclopentadienide
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. The cyclopentadienyl anion is a conjugated system because there are alternating π and σ bonds.
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.
Cyclopentadienyl complex
The cyclopentadienyl anion is a common ligand. 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. 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. The cyclopentadienyl anion has 5 atoms in an uninterrupted π system that bind to a metal, so it has a maximum hapticity of 5.
Official source
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