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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 ?
Two new metallocene linear MDPE rotomolding grades of 0.
Although market penetration of metallocene resins/elastomers varies with application, the aggregate total of its competitive materials is in the range of 20 to 25 billion lb/yr--hence the overall small penetration.
Additionally, the ability to accommodate Thailand's growing PE demand with proven LLDPE products as well as the ability to produce PE products made with metallocene technology were cited.
Metallocene polyolefins accounted for only about 2.
Additionally, the ability to accommodate China's growing PE demand with a broad range of polyethylene options, including advanced PE products made with metallocene technology, also was cited.
With the level of research spending and capital being put into metallocene technology, however, major penetration is expected to take place over the next decade.
Since the introduction of Vistamaxx specialty elastomers and the successful startup of a new metallocene elastomer plant, ExxonMobil Chemical has produced and sold commercial quantities of Vistamaxx in North America, Europe and Asia.
These are high molecular weight and high Mooney Nordel MG hydrocarbon rubber products from the company's metallocene gas phase EPDM process.
Having crossed the line between theory and practicality, into the real world of dynamic market competition, metallocene catalysts are pushing the plastics industry into the twenty-first century.
Incorporating two new, advanced Nexxstar(TM) HDPE and LDPE products developed expressly for high performance Nexxstar resin formulated shrink and Exceed(TM) metallocene linear low density PE, the technology has been tested on state-of-the-art packaging machinery at KHS-Kisters in Germany and at brand owners in Europe, North America, Africa and Asia.
bow will reclaim its Engage metallocene polyolefin elastomers (POE) and Tyrin CPEs, and will also acquire the former DuPont Nordel EPDM business, which has been enhanced via bow's Insite metallocene technology.
After the introduction of metallocene catalysts in the mid-1980s, it was suggested that metallocenes would revolutionize the polyolefin industry, but in reality, after almost 20 years of opportunity, only about 5% of the world's polyolefin production is made using metallocene catalysts (ref.