Cyclopentadienyl complex

A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups (C
5H−
5, abbreviated as Cp⁻). Based on the type of bonding between the metals and the cyclopentadienyl moieties, cyclopentadienyl complexes are classified into the following three categories: a) π-complexes, b) σ-complexes, and c) ionic complexes.

The alkali-metal cyclopentadienyl complexes react with various transition metal compounds to form a variety of complexes that are found throughout chemistry. The Cp ligand typically coordinates metals through the donation of π-electrons. This is typically drawn as dashed bonds to the metal center through all five carbon atoms. Examples include ferrocene, [FeCp₂]; the Fp dimer, [CpFe(CO)₂]₂; chromocene, [CrCp₂]; , [CoCp₂]; and nickelocene, [NiCp₂]. When the Cp rings are mutually parallel the compound is known as a sandwich complex. This area of organometallic chemistry was first discovered in 1954 with the determination of the structure of ferrocene Some metallocenes, notably rhodocene, [RhCp₂], and nickelocene, do exist in some forms where the metal-to-ring binding does not include all five ring atoms. Other metals form bent complexes, e.g., zirconocene dichloride, [ZrCp₂Cl₂], a catalyst for ethylene polymerization. Compounds featuring only one Cp ring are known as half-sandwich compounds or as piano stool compounds.

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