Cp2M

Cp2M is a general designation in organometallic chemistry for a family of sandwich complexes in which a metal center M is coordinated by two η5-cyclopentadienyl ligands (Cp). The Cp ligand is the cyclopentadienyl anion (C5H5−); in these complexes the two Cp rings are typically parallel, creating a classic “sandwich” geometry with the metal situated between them.

Electron counting in Cp2M complexes follows the 18-electron rule. Each Cp ligand donates six electrons, giving 12 electrons from the ligands; the metal center provides the remaining electrons to reach 18. Most Cp2M species are neutral and correspond to M in the +2 oxidation state, though the exact oxidation state can vary with additional ligands or substituents.

Representative members include ferrocene, Cp2Fe; cobaltocene, Cp2Co; nickelocene, Cp2Ni; vanadocene, Cp2V; titanocene, Cp2Ti; zirconocene, Cp2Zr; and hafnocene, Cp2Hf. Ferrocene is the archetype and was instrumental in establishing the existence of metallocenes; its discovery and subsequent structural characterization contributed to Nobel Prize-winning work in organometallic chemistry.

Synthesis of Cp2M compounds typically involves formation of cyclopentadienyl reagents and reaction with a suitable metal source, or the reductive coupling of Cp-containing precursors. In many cases Cp2M species serve as precursors to activated metallocene catalysts used in olefin polymerization, and they have broad use in studying metal–ligand bonding and redox chemistry.

See also: Ferrocene, Metallocene, Cyclopentadienyl ligand, Olefin polymerization catalysts.