Cyclopentadienyl complex
A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (C
5H−
5, 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.[1][2]
Examples
Biscyclopentadienyl complexes are called
Mixed-ligand Cp complexes containing Cp ligand and one or more other ligands. They are more numerous. One widely studied example is the
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.[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
Still rarer, the Cp unit can bond to the metal via three carbons. In these η3-Cp complexes, the bonding resembles that in
Moreover, inverse sandwich compounds with the "metal–Cp–metal" structures are known.[4]
Synthesis of Cp complexes
The compounds are generally prepared by
Most Cp complexes are prepared by substitution of preformed Cp complexes by replacement of halide, CO, and other simple ligands.
Variations of Cp complexes
-
Decamethylcobaltocene, a powerful reducing agent derived from "Cp*".
-
Bulky Cp ligandas found in (tBu3C5H2)2Fe2N2
Ansa Cp ligands
A pair of cyclopentadienyl ligands can be covalently linked giving rise to so-call ansa metallocenes. The angle between the two Cp rings is fixed. Rotation of the rings about the metal-centroid axis is stopped as well. A related class of derivatives give rise to the constrained geometry complexes. In these cases, a Cp ligand as linked to a non-Cp ligand. Such complexes have been commercialized for the production of polypropylene.
Bulky Cp ligands
Pentamethylcyclopentadiene gives rise to pentamethylcyclopentadienyl (Cp*) complexes. These ligands are more basic and more lipophilic. Replacing methyl groups with larger substituents results in cyclopentadienes that are so encumbered that pentaalkyl derivatives are no longer possible. Well-studied ligands of this type include C5R4H− (R = iso-Pr) and 1,2,4-C5R3H2− (R = tert-Bu).
Constrained geometry complexes
Constrained geometry complexes are related to ansa-metallocenes except that one ligand is not Cp-related.
Applications
Cp metal complexes are mainly used as stoichiometric reagents in chemical research. Ferrocenium reagents are oxidants. Cobaltocene is a strong, soluble reductant.
Derivatives of
References
- ^ ISBN 978-3-527-29390-2
- ^ Yamamoto, A. (1986). Organotransition Metal Chemistry: Fundamental Concepts and Applications. New York, NY: Wiley-Interscience. p. 105.[ISBN missing]
- ^ Crabtree, R. H. (2001). The Organometallic Chemistry of the Transition Metals (3rd ed.). New York, NY: John Wiley & Sons.[ISBN missing]
- S2CID 229180362.
Further reading
- Shriver, D.; Atkins, P. W. (1999). Inorganic Chemistry. New York, NY: W. H. Freeman.[ISBN missing]
- King, R. B.; Bisnette, M. B. (1967). "Organometallic chemistry of the transition metals XXI. Some π-pentamethylcyclopentadienyl derivatives of various transition metals". J. Organomet. Chem. 8 (2): 287–297. . [Initial examples of the synthesis of Cp*-metal complexes]