A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (, abbreviated as Cp−). Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (η5-) 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 (FeCp2), which has many analogues for other metals, such as chromocene (CrCp2), cobaltocene (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 developed in the 1950s. Bent metallocenes are represented by compounds of the type [MCp2Lx]. 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, (Cp2Fe2(CO)4). 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)3). Bonding modes All 5 carbon atoms of a Cp ligand are bound to the metal in the vast majority of M–Cp complexes. This bonding mode is called η5-coordination. The M–Cp bonding arises from overlap of the five π molecular orbitals of the Cp ligand with the s, p, and d orbitals on the metal. These complexes are referred to as π-complexes. Almost all of the transition metals employ this coordination mode. 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 CpSiMe3. An example of both is (Cp2Fe(CO)2). It is probable that η1-Cp complexes are intermediates in the formation of η5-Cp complexes. Still rarer, the Cp unit can bond to the metal via three carbons. In these η3-Cp complexes, the bonding resembles that in allyl ligands. Such complexes, sometimes called "slipped Cp complexes", are invoked as intermediates in ring slipping reactions. Moreover, inverse sandwich compounds with the "metal–Cp–metal" structures are known. Synthesis of Cp complexes The compounds are generally prepared by salt metathesis reactions of alkali-metal cyclopentadienyl compounds with transition