metallocenen

Metallocenes are a class of organometallic compounds in which a metal center is sandwiched between two cyclopentadienyl ligands. In the classic formulation, the rings are bound to the metal in an η5 fashion, giving the general formula (C5H5)2M, where M is a transition metal. Substituted cyclopentadienyl rings, such as Cp* (C5Me5), and other ligand arrangements give rise to a broad family of metallocene compounds.

The most well-known metallocenes are sandwich complexes of iron, cobalt, nickel, and other metals, including ferrocene Fe(C5H5)2, cobaltocene Co(C5H5)2, nickelocene Ni(C5H5)2, and ruthenocene Ru(C5H5)2. Early metallocenes often conform to an 18-electron rule, with the two Cp rings donating a total of 12 electrons to the metal, though deviations occur for certain metals and oxidation states. The structures are typically highly symmetrical and exhibit facile Cp-ring rotation that does not destroy the sandwich arrangement.

Ferrocene, discovered in 1951 by Pauson and Kealy, catalyzed a revolution in organometallic chemistry. The discovery, and the subsequent development of related metallocenes, earned Geoffrey Wilkinson, Ernst Fischer, and Paul Pauson a Nobel Prize in Chemistry in 1973. Metallocenes also include derivatives such as titanocene dichloride and zirconocene dichloride, which have been explored as catalysts.

Applications of metallocenes span fundamental studies in bonding and organometallic chemistry to practical uses in catalysis, particularly in polymerization of olefins and related processes. Cp2M scaffolds and their Cp* variants have become central to single-site catalyst technologies in industry.