A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups (C5H5-, 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.
a) π-complexes
Metals and cyclopentadienyl moieties connected by π-bonds are called π-complexes. π-Complexes, especially in the η5-type coordination mode, are the most typical of the three types of complexes. Almost all of the transition metals, that is, group 4 to 10 metals, employ this coordination mode. This type of cyclopentadienyl complex is mainly synthesized by a reaction of metal halide with sodium cyclopentadienide (CpNa), trimethylsilyl cyclopentadiene (CpSiMe3), or thallium cyclopentadienide (CpTl) in an ether-type solvent, or with cyclopentadiene in the presence of a base. These complexes can have various kinds of other ligands besides a cyclopentadienyl ligand, such as carbonyl, halogen, alkyl, and so on. In particular, biscyclopentadienyl complexes are called metallocenes. It is often the case that these complexes are thermally stable, and they are used for various catalysts. For example, some of early transition metal complexes, such as Cp2TiCl2 and Cp2ZrCl2 with methylaluminoxane as a co-catalyst, can work well as catalysts for olefin polymerization reactions; these are called Kaminsky-type catalysts.
b) σ-complexes
σ-Complexes have a direct σ-bond between the metal and one of the carbons of the cyclopentadienyl group. Typical examples of this type of complex are group 14 metal complexes such as CpSiMe3, Cp2Sn, and CpPb. CpSiMe3 is commonly used as the starting material for the synthesis of group 4 metal cyclopentadienyl complexes.
c) Ionic complexes
Ionic complexes primarily occur with alkali metal cations and alkali earth metals cations connected to cyclopentadienyl anions. Ionic type complexes are synthesized generally by a reaction of cyclopentadiene with a metal in an non-aromatic solvent. These complexes can be good starting materials for several π-type cyclopentadienyl complexes.