Metallocenes
Authors: Bond Gary P. 
Published: August 2005

Metallocenes are organometallic complexes with a central metal atom attached to aromatic ligands. Group II iron, which has a 2+ valence, was in one of the earliest metallocenes, but group IV metals such as titanium are currently more common. For group IV metallocenes, halides (e.g., chlorine), pseudohalides (e.g., carboxylates), or other similar molecules can also be attached to the metal. There is some debate among metallocene scientists whether one or two aromatic ligands are required for the complex to be a metallocene, but it is currently generally accepted that two aromatic ligands attached to the metal are a basic definitional criterion for metallocenes.

Metallocenes have been used as chemical intermediates, antiknock additives to gasoline, lubricants, and for other uses, but the main current application is as catalysts in the plastics industry. Metallocenes are also currently being actively investigated as a cancer treatment agent. Catalysts, substances that initiate or speed up desirable chemical reactions, have made possible the development of several modern plastic polymers. Metallocene catalysts are improved over the initial plastics polymerization catalysts of the 1950s of Karl Ziegler and Giulio Natta (Ziegler–Natta catalysts), which catalyzed polymerization of ethylene and propylene into polyethylene and polypropylene. Metallocenes provide better control of polymerization, making it possible to create plastics with physical properties designed for particular uses. For example, precise control over polymer growth allows making plastics that are durable for gearwheels or can withstand high temperatures for piping. The billions of pounds of plastics produced and sold every year can be light, waterproof, resistant to corrosion, and used in such items as water pipes, trash bags, hair combs, fibers for clothing and road construction, and packaging for food and medicines, just to name a few uses.

Metallocene catalysts, first synthesized in 1953 by John Birmingham, contain a metal. Unlike the Ziegler–Natta catalysts, metallocenes contain just a single atom of metal. Because of the widespread production of plastics via metallocene catalysts, metallocenes have become worthy of consideration for potential effects on the worker and the workplace. The antitumor effects of metallocenes underscore the importance of this consideration for protecting workers from workplace chemicals that interact with nuclear material and the cellular duplication process. Two important examples of metallocene catalysts discussed in this chapter are ferrocene and titanocene. Ferrocene, an atypical group II metallocene that has iron (Fe) as the central metal with two cyclopentadiene cyclic groups has no halide groups and is considered first because it was one of the early metallocenes. Ferrocene is the first metallocene to have the “sandwich” structure created by two cyclopentadiene rings attached to the metal. Titanocene, with group IV titanium as the metal, has two cyclopentadiene rings and two chlorine molecules attached to the metal. Little toxicological data are available at present for most metallocenes and limits the number of metallocenes covered in this chapter. If metallocenes are developed into anticancer agents for humans, the amount of available toxicological data will certainly increase. The initial chemical listed, dicyclopentadiene, although not a metallocene, is presented as a baseline for its health effects, and the intention is to compare it to dicyclopentadiene iron and the other metallocenes presented.