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(organic chemistry)
Organometallic coordination compound which is obtained as a cyclopentadienyl derivative of a transition metal or a metal halide.



any of the dicyclopentadienyl derivatives of transition metals and metal halides. Metallocenes have the general formula (C5H5)2M and a “sandwich” structure, in which the metal atom, in a +2 oxidation state, is sandwiched between the C5H5 rings and bonded to all ten carbon atoms by covalent bonds.

Metallocenes are known for almost all the transition metals. In the most stable metallocenes—ferrocene, ruthocene, and osmocene—the metal atom has the electronic configuration of an inert gas. These metallocenes display aromatic properties. The physical and chemical properties of ferrocene have been more widely studied than those of other metallocenes. The metallocenes of such metals as titanium, niobium, and tantalum are unstable and are isolated only as derivatives with the general formula (C5H5)2MXn, where n = 1, 2, or 3 and “X” stands for H or one of the halogens, such as Cl or Br. In contrast to other metallocenes, manganocene has an ionic structure, (C5H5)2 Mn2+; such metallocenes as chromocene, (C5H5)2Cr, and vanadocene, (C5H5)2V, are partially ionic.

Metallocenes are intensely colored, crystalline compounds. They are readily soluble in organic solvents. In acidic aqueous solutions they are easily oxidized to cations with the formula (C5H5)2M+. The properties of these cations are similar to those of large, singly charged metal cations, such as Cs+. The metallocenes are precipitated as, for example, silicotungstate, chloroplatinate, or tetraphenylborate. The cobaltocene cation, (C5H5)2Co+, is not destroyed by concentrated nitric or sulfuric acid, even upon heating.

Metallocenes are produced by the action of salts or acetylacetonates of transition metals on sodium cyclopentadienide, (C5H5)Na+, or on the cyclopentadienyl magnesium halide, C5H5MgBr. They may also be produced by the action of cyclopentadiene on a transition metal salt (in the presence of a base) or on a carbonyl.

Such metallocenes as (C5H5)2TiCl2 are used as cocatalysts in homogeneous catalytic reactions. Other metallocenes are used as drugs; for example, a ferrocene derivative, ferrocerone, C5H5FeC5H4COC6H4COONa-ortho·4H2O, is used for the treatment of diseases caused by an iron deficiency.


References in periodicals archive ?
Including all metallocenes in it is not correct," says Kenneth Sinclair, principal of polymer consultant STA+Research in Lucey, Wash.
The article describes a revival in the use of metallocenes and expanded marketing of super-strong polyethylene plastics.
One area where metallocenes have made a significant impact is in the low-density, ethylene elastomer (EE) markets which include plastomers (ethylene [alpha]-olefin copolymers with density less than 0.910 g/cc) and EP(D)M products.
It required a fourth extruder and a five-layer feedblock and allowed use of less-expensive butene LLDPE reinforced with thin support layers of thennew metallocene LLDPE.
Technology-sharing agreements, like the metallocene cooperation between Basell and ExxonMobil, should also be powerful engines for innovation.
Metallocene grades are taking a wider role, but they are only one part of the story.
Metallocene based polyethylene (PE) plastomers, PE elastomers and enhanced polyethylene (partially metallocene catalyzed PEs) are currently used in crosslinked polyethylene (XLPE) based foams in sporting goods and recreational applications, automotive and medical applications.
"Competition for metallocenes could turn ugly, " Plastics World, January 1995.
Olefinic TPEs, SEBS compounds, and, more recently, metallocene plastomers compete for market share at the plastic/elastomer interface and for a portion of the potential promised by the substitution for PVC.
When it comes to polyethylene innovation, metallocenes are far from the only game in town.
This phenoxyimine complex reportedly has 10 times higher ethylene-polymerization activity than current metallocenes and produces resins with low to ultra-high molecular weight.
Metallocenes are currently being employed by Dow in the development of ethylene-styrene copolymers ranging from low styrene content to pure syndiotactic polystyrene.