A metallocene is a compound typically consisting of two cyclopentadienyl anions (Cp, which is C5H5-) bound to a metal center (M) in the oxidation state II, with the resulting general formula (C5H5)2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+ catalyze olefin polymerization. Metallocenes are a subset of a broader class of organometallic compounds called sandwich compounds.
In the structure shown at right, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they are aromatically stabilized. Here they are shown in a staggered conformation.
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History [link]
Ferrocene was considered to be the first metallocene, discovered in 1951 by Pauley and Kealy at Duquesne University. The two accidentally discovered the compound but were amazed by the compounds stability.[1]. Metallocene's played a factor in more research being pursued in organometallic chemistry.This research has led to research being done in the polyethylene and polypropylene markets. The research being done in these fields will help new classes of polymers.
Definition [link]
The general name metallocene is derived from ferrocene, (C5H5)2Fe or Cp2Fe , systematically named bis(η5-cyclopentadienyl)iron(II). According to the IUPAC definition, a metallocene contains a transition metal and two cyclopentadienyl ligands coordinated in a sandwich structure, i. e., the two cyclopentadienyl anions are co-planar with equal bond lengths and strengths. Using the nomenclature of "hapticity", the equivalent bonding of all 5 carbon atoms of a cyclopentadienyl ring is denoted as η5, pronounced "pentahapto." There are exceptions, such as uranocene, which has two cyclooctatetraene rings sandwiching a uranium atom.
In metallocene names, the prefix before the -ocene ending indicates what metallic element is between the Cp groups. For example in ferrocene, iron(II) or ferrous is present.
In contrast to the more strict definition proposed by IUPAC, which requires a d-block metal and a sandwich structure, the term metallocene and thus the denotation -ocene, is applied in the chemical literature also to non-transition metal compounds, such as Cp2Ba, or structures where the aromatic rings are not co-planar, such as found in manganocene or titanocene dichloride (Cp2TiCl2).
Some metallocene complexes of actinides have been reported where there are three cyclopendadienyl ligands for a monometallic complex, all three of them bound η5.[2]
Synthesis, properties, and structures of metallocenes [link]
Most metallocenes are prepared by the reaction of sodium cyclopentadienide with metal dihalides:
- MCl2 + 2 NaC5H5 → M(C5H5)2 + 2 NaCl
Many other methods have been developed. Chromocene can be prepared from chromium hexacarbonyl by direct reaction with cyclopentadiene in the presence of diethylamine; in this case, the formal deprotonation of the cyclopentadiene is followed by reduction of the resulting protons to hydrogen gas, facilitating the oxidation of the metal centre.[3]
- Cr(CO)6 + 2 C5H6 → Cr(C5H5)2 + 6 CO + H2
Metallocenes generally have high thermal stability. Ferrocene can be sublimed in air at over 100 °C with no decomposition; metallocenes are generally purified by vacuum sublimation. Charge-neutral metallocenes are soluble in common organic solvents. Alkyl substituted derivative are particularly soluble, even in alkane solvents.
Structure [link]
A structural trend for the series MCp2 involves the variation of the M-C bonds, which elongate as the valence electron count deviates from 18.[4]
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M(C5H5)2 rM-C (pm) valence electron count Fe 203.3 18 Co 209.6 19 Cr 215.1 16 Ni 218.5 20 V 226 15
In metallocenes of the type (C5R5)2M, the cyclopentadienyl rings rotate with very low barriers. Single crystal X-ray diffraction studies reveal both eclipsed or staggered rotamers. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a few kJ/mol. Crystals of ferrocene and osmocene exhibit eclipsed conformations at low temperatures, whereas in the related bis(pentamethylcyclopentadienyl) complexes the rings usually crystallize in a staggered conformation, apparently to minimize steric hindrance between the methyl groups.
Derivatives [link]
- Ansa-metallocenes: Derivatives of metallocenes include structures with an intramolecular bridge between the two cyclopentadienyl rings (ansa-metallocenes)
- Triple decker complexes: compounds with three Cp anions and two metal cations in alternating order, e.g. [Ni2Cp3]+.
- Metallocenium cations: the most famous example is ferrocenium, [Fe(C5H5)2]+, derived from oxidation of ferrocene (few metallocene anions are known).
See also [link]
Footnotes [link]
- ^ T. J. Kealy, P. L. Pauson (1951). "A New Type of Organo-Iron Compound". Nature 168 (4285): 1039. DOI:10.1038/1681039b0.
- ^ Brennan, John G.; Andersen, Richard A.; Zalkin, Allan (1986). "Chemistry of trivalent uranium metallocenes: Electron-transfer reactions. Synthesis and characterization of [(MeC5H4)3U]2E (E = S, Se, Te) and the crystal structures of hexakis(methylcyclopentadienyl)sulfidodiuranium and tris(methylcyclopentadienyl)(triphenylphosphine oxide)uranium". Inorg. Chem. 25 (11): 1761–1765. DOI:10.1021/ic00231a008.
- ^ Fischer, E. O.; Hafner, W. Z. (1955). "Cyclopentadienyl-Chrom-Tricarbonyl-Wasserstoff" (in German). Z. Naturforsch B. 10 (3): 140–143.
- ^ Kevin R. Flower, Peter B. Hitchcock "Crystal and molecular structure of chromocene (η5-C5H5)2Cr" Journal of Organometallic Chemistry, 1996, volume 507, p. 275-277. doi:10.1016/0022-328X(95)05747-D. Discusses all metallocene structures available at that time.
Additional references [link]
- A. Salzer (1999). "Nomenclature of Organometallic Compounds of the Transition Elements". Pure Appl. Chem. 71 (8): 1557–1585. DOI:10.1351/pac199971081557. https://www.iupac.org/reports/1999/7108salzer/index.html.
- Robert H. Crabtree The Organometallic Chemistry of the Transition Metals 4th ed. Wiley-Interscience: 2005.
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