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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 ?
Ziegler Natta catalysts still dominate the Polypropylene Catalyst Market with Metallocene Catalysts set to register the fastest growth rate in the market during 2015-2020"
Our reliable, affordable machines combined with film formulations based on ExxonMobil Chemical s metallocene resins and elastomers deliver excellent performance, high speed and low production costs.
After nearly 30 years of operating a UNIPOL PE plant in South Africa, Sasol has a long-standing relationship with Univation, and Sasol is looking forward to expanding its product portfolio to include latest generation Univation metallocene technologies," commented Mike Biesheuvel, Managing Director, Sasol Polymers International Investments.
Compared to conventional polyolefins, Lumicene [R] metallocene polyolefins offer enhanced performance in terms of environmental properties, gloss and transparency, gas permeability, chemical resistance, mechanical properties and processability They are based on the latest generation of 'single site' metallocene catalysts used in the polyolefin polymerisation processes, and are the result of continuous investment in innovation by Total Petrochemicals.
Total's metallocene PP resin clarified with Millad NX8000 agent offers a new and improved choice for clear containers and packaging.
This company is said to offer customers one of the industry's broadest portfolios of specialty elastomer products, including conventional and metallocene catalysts, TPVs, specialty elastomers, etc.
Consequently, the relaxation of metallocene aPPs is perfectly identifiable as the glass-rubber transition in a narrow molecular weight amorphous polymer (14).
The primary advantages metallocene catalyst technology holds over conventional Ziegler/Natta processes in polyolefins production are versatility and control.
The metallocene used for the innermost layer is formulated for improved processability and thus allows faster filling.
The report, " Polypropylene Catalysts Market by Type (Ziegler - Natta, Metallocene, and Co-Catalyst systems), by PP Manufacturing Process (Solvent, Bulk Slurry, and Gas phase) - Global trends & forecasts to 2019 " , published by MarketsandMarkets, Global Polypropylene Catalysts Market is estimated at $ 840 Million in 2014 and projected to reach a market size of $1,123 Billion in 2019 registering a CAGR of 5% - 6% between the same periods.
Patent 7,807,096 (October 5, 2010), "Injection Blow Molded Single-Layer Metallocene Polyethylene Container," Eric Maziers (Total Petrochemicals Research Feluy, Seneffe, Feluy, Belgium).
A new metallocene PE grade for tough, stiff, high clarity films has been added to Exxon Chemical's metallocene PE range.