Metallocenes
Metallocenes are a type of organometallic complex, i.e., a chemical compound in which a metal atom is bonded to an organic structure. With the metallocenes, one or more aromatic rings are bonded to the metal ion by their pi electrons (electrons that occur in a certain type of orbit around the atom or molecule).
Ferrocene was the first metallocene to be discovered. This reddish-orange crystalline solid melts at 343°F (173°C). It is also one of a subgroup called the "sandwich compounds" because its two aromatic rings are parallel to each other--one above and one below the plane containing the metal atom (in this case, iron).
Metallocenes are formed by the combination of ionic cyclopentadiene, a reactive but aromatic organic anion, with derivatives of the transition metals or metal halides. (Transition metals are divided into three main series and include chromium, cobalt, hafnium, iron, titanium, vanadium, ruthenium, rhodium, zirconium, tungsten, molybdenum, osmium, and nickel. Halides, in this case, are compounds of halogens with another element.) Ionic cyclopentadiene's aromatic rings coordinate with the positive ions in the transition metals, forming a metallocene.
Metallocenes are known for their vibrant colors. For instance, those based on titanium are green, those made of chromium (chromocene) are red, cobalt metallocenes (cobaltocene) are purple, and osmium metallocenes are yellow. Some of these metallocenes are more reactive than others in terms of stability. For instance, scientists must be extremely careful when dealing with chromocene, since, when finely powdered, it tends to spontaneously burst into flame after a few moments in the open air. The compounds also tend to be paramagnetic, meaning their magnetic moments can be aligned in the direction of the applied field. Only those metallocenes in the iron group (ferrocene, ruthenium, and osmium) are dimagnetic, which means that their magnetization is in the opposite direction of the applied field.
Another property of metallocenes is that they are easily oxidized to cation (positive ion) forms of transition metals. Cobalticinium and ferricinium are examples. In addition, when the central metal atom of a metallocene is in a stable oxidative state (i.e., its ion has lost or gained all the electrons it is going to), the metallocene is impervious to decomposition by air, high temperatures, bases, dilute acids, or water. Otherwise, however, many metallocenes are unstable when exposed to air, heat, or water.
Metallocenes are useful in industrial chemistry as reducing agents (deoxidizers), anti-knock agents for internal combustion engine fuels, absorbers of ultraviolet light, and free radical scavengers. However, perhaps their most important use is as catalysts in creating organic compounds. For example, they are central to the most common industrial method for making acetic acid, which is the main ingredient in vinegar and essential in the manufacture of some solvents, acetate fibers, and flavors. New metallocene catalysts are used in the preparation of new plastics, including new methods for the polymerization of alkenes such as ethylene and propylene.
This is the complete article, containing 475 words
(approx. 2 pages at 300 words per page).
