metal chlorides. Sodium cyclopentadienide (NaCp) and lithium cyclopentadienide are commonly used. Trimethylsilylcyclopentadiene cyclopentadienylthallium (CpTl) are alternative sources. For the preparation of some particularly robust complexes, e.g. nickelocene, cyclopentadiene is employed in the presence of a conventional base such as KOH. When only a single Cp ligand is installed, the other ligands typically carbonyl, halogen, alkyl, and hydride. Most Cp complexes are prepared by substitution of preformed Cp complexes by replacement of halide, CO, and other simple ligands. Variations of Cp complexes Ansa Cp ligands A pair of cyclopentadienyl ligands can be covalently linked giving rise to so-call ansa metallocenes. The angle between the two Cp rings is fixed. Rotation of the rings about the metal-centroid axis is stopped as well. A related class of derivatives give rise to the constrained geometry complexes. In these cases, a Cp ligand as linked to a non-Cp ligand. Such complexes have been commercialized for the production of polypropylene. Bulky Cp ligands Pentamethylcyclopentadiene gives rise to pentamethylcyclopentadienyl (Cp*) complexes. These ligands are more basic and more lipophilic. Replacing methyl groups with larger substituents results in cyclopentadienes that are so encumbered that pentaalkyl derivatives are no longer possible. Well-studied ligands of this type include C5R4H− (R = iso-Pr) and 1,2,4-C5R3H2− (R = tert-Bu). Constrained geometry complexes Constrained geometry complexes are related to ansa-metallocenes except that one ligand is not Cp-related. Applications Cp metal complexes are mainly used as stoichiometric reagents in chemical research. Ferrocenium reagents are oxidants. Cobaltocene is a strong, soluble reductant. Derivatives of Cp2TiCl2 and Cp2ZrCl2 are the basis of some reagents in organic synthesis. Upon treatment with aluminoxane, these dihalides give catalysts for olefin polymerization. Such species are called Kaminsky-type catalysts. References Further reading [Initial examples of the synthesis of Cp*-metal complexes] Organometallic chemistry