thumb|right|200px|[[Zirconocene dichloride, a cyclopentadienyl complex]]
A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (, abbreviated as Cp<sup>−</sup>). Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (η<sup>5</sup>-) bonding mode. The metal–cyclopentadienyl interaction is typically drawn as a single line from the metal center to the center of the Cp ring.
Examples
Biscyclopentadienyl complexes are called metallocenes. A famous example of this type of complex is ferrocene (FeCp<sub>2</sub>), which has many analogues for other metals, such as chromocene (CrCp<sub>2</sub>), cobaltocene (CoCp<sub>2</sub>), and nickelocene (NiCp<sub>2</sub>). When the Cp rings are mutually parallel the compound is known as a sandwich complex. This area of organometallic chemistry was first developed in the 1950s. Bent metallocenes are represented by compounds of the type [MCp<sub>2</sub>L<sub>x</sub>]. Some are catalysts for ethylene polymerization. Metallocenes are often thermally stable, and find use as catalysts in various types of reactions.
Mixed-ligand Cp complexes containing Cp ligand and one or more other ligands. They are more numerous. One widely studied example is the Fp dimer, (Cp<sub>2</sub>Fe<sub>2</sub>(CO)<sub>4</sub>). Monometallic compounds featuring only one Cp ring are often known as half sandwich compounds or as piano stool compounds, one example being methylcyclopentadienylmanganese tricarbonyl (CpMn(CO)<sub>3</sub>).
Bonding modes
In the vast majority of M–Cp complexes, all 5 carbon atoms of a Cp ligand bind the metal (η<sup>5</sup>-coordination). The M–Cp bonding arises from overlap of the five molecular orbitals in the Cp π system with the metal s, p, and d orbitals; hence these complexes are referred to as π-complexes. For example, bis(trispyrazolyl)iron(II) is high spin, whereas ferrocene is low spin.
In relatively rare cases, Cp binds to metals via only one carbon center. These types of interactions are described as σ-complexes because they only have a σ bond between the metal and the cyclopentadienyl group. Typical examples of this type of complex are group 14 metal complexes such as CpSiMe<sub>3</sub>. An example of both is (Cp<sub>2</sub>Fe(CO)<sub>2</sub>). It is probable that η<sup>1</sup>-Cp complexes are intermediates in the formation of η<sup>5</sup>-Cp complexes.
Still rarer, the Cp unit can bond to the metal via three carbons. In these η<sup>3</sup>-Cp complexes, the bonding resembles that in allyl ligands. Such complexes, sometimes called "slipped Cp complexes", are invoked as intermediates in ring slipping reactions.
Synthesis of Cp complexes
The compounds are generally prepared by salt metathesis reactions of alkali-metal cyclopentadienyl compounds with transition metal chlorides. Sodium cyclopentadienide (NaCp) and lithium cyclopentadienide are commonly used. Trimethylsilylcyclopentadiene and cyclopentadienylthallium (CpTl) are alternative sources.