métallocènes

Metallocenes (métallocènes in French) are organometallic compounds in which a central metal atom is bound to two cyclopentadienyl ligands in a sandwich-like geometry. The simplest and most studied examples have the formula (C5H5)2M, where M is typically a transition metal such as iron, cobalt, or nickel in the +2 oxidation state, though early metallocenes with titanium, zirconium, hafnium, and others are also known. Ferrocene, Fe(C5H5)2, is the prototypical member and was instrumental in establishing the structural concept that the Cp rings donate six electrons to the metal in an η5 manner, giving an 18-electron configuration for many isolates.

In metal complexes with two η5-C5H5 ligands, the Cp rings are permanently bonded to the metal, forming a robust sandwich. Unsubstituted metallocenes often adopt a staggered arrangement (D5d symmetry) but some substituted variants can be eclipsed or exhibit lower symmetry. Substitution on the Cp rings or changes in the metal can alter redox properties and reactivity, enabling diverse applications.

Synthesis typically involves reaction of cyclopentadienyl anions with metal precursors, or reductive coupling of Cp-based ligands to give (C5H5)2M species. A major area of application is catalysis: metallocenes and related “single-site” catalysts, such as titanocene dichloride and zirconocene dichloride, are used in olefin polymerization when paired with suitable cocatalysts (e.g., MAO or boranes). Metallocenes also serve as useful models for bonding in organometallic chemistry and have been employed in asymmetric catalysis when chiral Cp ligands are introduced.

Historically, ferrocene was discovered in 1951, revealing a stable, reversible sandwich structure and launching the field of metallocene chemistry.