Cyclopentadienyl complex

Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (η5-) bonding mode.

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).

All 5 carbon atoms of a Cp ligand are bound to the metal in the vast majority of M–Cp complexes.

[1] 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.

[1] For the preparation of some particularly robust complexes, e.g. nickelocene, cyclopentadiene is employed in the presence of a conventional base such as KOH.

A pair of cyclopentadienyl ligands can be covalently linked giving rise to so-call ansa metallocenes.

A related class of derivatives give rise to the constrained geometry complexes.

Replacing methyl groups with larger substituents results in cyclopentadienes that are so encumbered that pentaalkyl derivatives are no longer possible.

Cp metal complexes are mainly used as stoichiometric reagents in chemical research.