Editing Cyclopentadienyl complex

{{Short description|Coordination complex of a metal and cyclopentadienyl groups}}
[[File:Zirconocene-dichloride-3D-sticks.png|thumb|right|200px|[[Zirconocene dichloride]], a '''cyclopentadienyl complex''']]

A '''cyclopentadienyl complex''' is a [[coordination complex]] of a [[metal]] and [[cyclopentadienyl anion|cyclopentadienyl groups]] ({{chem|C|5|H|5|−}}, abbreviated as '''Cp<sup>−</sup>'''). Cyclopentadienyl ligands almost invariably bind to metals as a [[hapticity|pentahapto]] (''η''<sup>5</sup>-) bonding mode.  The metal–cyclopentadienyl interaction is typically drawn as a single line from the metal center to the center of the Cp ring.<ref name=Elsch>Elschenbroich, C. "Organometallics" (2006) Wiley-VCH: Weinheim. {{ISBN|978-3-527-29390-2}}</ref><ref>{{cite book|first=A. |last=Yamamoto |date=1986 |title=Organotransition Metal Chemistry: Fundamental Concepts and Applications |location=New York, NY |publisher=Wiley-Interscience |page=105}}{{ISBN missing}}</ref>

==Examples==
''Bis''cyclopentadienyl complexes are called [[metallocene]]s. A famous example of this type of complex is [[ferrocene]] (FeCp<sub>2</sub>), which has many analogues for other metals, such as [[chromocene]] (CrCp<sub>2</sub>), [[cobaltocene]] (CoCp<sub>2</sub>), and [[nickelocene]] (NiCp<sub>2</sub>). When the Cp rings are mutually parallel the compound is known as a [[sandwich compound|sandwich]] [[coordination complex|complex]]. This area of [[organometallic chemistry]] was first developed in the 1950s.  Bent metallocenes are represented by compounds of the type [MCp<sub>2</sub>L<sub>x</sub>].  Some are catalysts for [[ethylene]] [[polymerization]].<ref>{{cite book|first=R. H. |last=Crabtree |date=2001 |title=The Organometallic Chemistry of the Transition Metals |edition=3rd |location=New York, NY|publisher= John Wiley & Sons}}{{ISBN missing}}</ref> Metallocenes are often thermally stable, and find use as [[catalyst]]s in various types of reactions.

Mixed-ligand Cp complexes containing Cp ligand and one or more other ligands.  They are more numerous. One widely studied example is the [[cyclopentadienyliron dicarbonyl dimer|Fp dimer]], (Cp<sub>2</sub>Fe<sub>2</sub>(CO)<sub>4</sub>). Monometallic compounds featuring only one Cp ring are often known as [[half sandwich compound]]s or as piano stool compounds, one example being [[methylcyclopentadienylmanganese tricarbonyl]] (CpMn(CO)<sub>3</sub>).

==Bonding modes==
In the vast majority of M–Cp complexes, all 5 carbon atoms of a Cp ligand bind the metal ([[Hapticity|''η''<sup>5</sup>-coordination]]). The M–Cp bonding arises from overlap of the five molecular orbitals in the Cp [[conjugated system|π system]] with the metal ''s'', ''p'', and ''d'' orbitals; hence these complexes are referred to as π-complexes.<Ref name=Elsch/>  ''η''<sup>5</sup>-coordinated Cp is [[isolobal]] to three facial carbonyls,<ref>{{cite book|pp=387-388|year=1985|publisher=Wiley|lccn=84-15310|isbn=0-471-87393-4|last1=Albright|first1=T.&nbsp;A.|last2=Burdett|first2=J.&nbsp;K.|author3=Whangbo Myung-Hwan|title=Orbital Interactions In Chemistry}}</ref> as well as [[scorpionate ligand]]s and thia-[[crown ether]]s.{{Cn|date=May 2025}}  Almost all of the [[transition metal]]s employ this coordination mode.<ref name=Elsch/>

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. Typical examples of this type of complex are group 14 metal complexes such as CpSiMe<sub>3</sub>. An example of both is (Cp<sub>2</sub>Fe(CO)<sub>2</sub>).  It is probable that ''η''<sup>1</sup>-Cp complexes are intermediates in the formation of ''η''<sup>5</sup>-Cp complexes.

Still rarer, the Cp unit can bond to the metal via three carbons. In these ''η''<sup>3</sup>-Cp complexes, the bonding resembles that in [[allyl ligand]]s. Such complexes, sometimes called "slipped Cp complexes", are invoked as intermediates in [[indenyl effect|ring slipping reactions]].

==Synthesis of Cp complexes==
The compounds are generally prepared by [[salt metathesis reaction]]s of alkali-metal cyclopentadienyl compounds with transition metal chlorides.  Sodium cyclopentadienide (NaCp) and [[lithium cyclopentadienide]] are commonly used. [[Trimethylsilylcyclopentadiene]] [[cyclopentadienylthallium]] (CpTl) are alternative sources.<ref name=Elsch/>  For the preparation of some particularly robust complexes, e.g. nickelocene, [[cyclopentadiene]] is employed in the presence of a conventional [[base (chemistry)|base]] such as KOH. When only a single Cp ligand is installed, the other [[ligand]]s typically carbonyl, halogen, alkyl, and hydride.

Most Cp complexes are prepared by substitution of preformed Cp complexes by replacement of halide, CO, and other simple ligands.

==Variations of Cp complexes==
<gallery caption="Variations of Cyclopentadienyl complexes" widths="160px" heights="100px">
File:Decamethylcobaltocene.svg|[[Decamethylcobaltocene]], a powerful reducing agent derived from "Cp*".
File:ConstrainedGeomCmpx.png|A [[Constrained geometry complex|constrained geometry organotitanium complex]]
File:Ansa-Metallocene.svg|An [[ansa-metallocene]]
File:Cp"2Fe2N2.png|[[Bulky Cp ligand]] as found in (<sup>''t''</sup>Bu<sub>3</sub>C<sub>5</sub>H<sub>2</sub>)<sub>2</sub>Fe<sub>2</sub>N<sub>2</sub>
</gallery>

===Ansa Cp ligands===
{{main|Ansa-metallocene}}
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 complex]]es.  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===
{{main|Bulky cyclopentadienyl 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 C<sub>5</sub>R<sub>4</sub>H<sup>−</sup> (R = iso-Pr) and 1,2,4-C<sub>5</sub>R<sub>3</sub>H<sub>2</sub><sup>−</sup> (R = ''tert''-Bu).

===Constrained geometry complexes===
{{main|constrained geometry complex}}
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 [[Titanocene dichloride|Cp<sub>2</sub>TiCl<sub>2</sub>]] and [[zirconocene dichloride|Cp<sub>2</sub>ZrCl<sub>2</sub>]] are the basis of some reagents in [[organic synthesis]]. Upon treatment with [[aluminoxane]], these dihalides give catalysts for [[Ziegler–Natta|olefin polymerization]]. Such species are called [[Kaminsky catalyst|Kaminsky-type catalysts]].

==References==
{{Reflist}}

==Further reading==
*{{cite book|first1=D. |last1=Shriver |first2=P. W. |last2=Atkins |date=1999 |title=Inorganic Chemistry |url=https://archive.org/details/inorganicchemist00shri |url-access=registration |location=New York, NY |publisher=W. H. Freeman}}{{ISBN missing}}
*{{cite journal|first1=R. B. |last1=King |first2=M. B. |last2=Bisnette |date=1967 |journal=J. Organomet. Chem. |volume=8|pages=287–297|title=Organometallic chemistry of the transition metals XXI. Some π-pentamethylcyclopentadienyl derivatives of various transition metals|issue=2 |doi=10.1016/S0022-328X(00)91042-8}} [Initial examples of the synthesis of Cp*-metal complexes]
{{Coordination complexes}}
{{Organometallics}}

[[Category:Cyclopentadienyl complexes|*]]
[[Category:Organometallic chemistry]]