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A positive step for silicon chemistry.

Scientists have made a novel form of silicon with potentially new properties to tap. A team of chemists at Northwestern University in Evanston, Ill. and Indiana University in Bloomington has created and identified the first stable silicon cation, a positively charged ion that has three bonds. Silicon naturally forms two, four, or five bonds.

The new silicon cation may lead to better strategies for synthesizing silicon-based materials, researchers believe.

For the past 50 years, chemists have aspired to make a form of silicon with three bonds. They believed this was possible because carbon, a close relative on the periodic table, can form a "carbocation," which also has three bonds and a positive charge. The highly reactive carbocation -- an intermediate in many organic and biological processes - has helped researchers probe the mechanisms behind chemical reactions. Carbocations also serve as industrial catalysts.

But the silicon cation proved more difficult to make than the carbocation. It is more reactive and thus more fleeting. In the June 25 SCIENCE, the researchers describe how they trapped this elusive chemical species.

Led by Joseph B. Lambert of Northwestern University, the group made two key breakthroughs this past year. First, they discovered a good solvent that stabilized the cation. Toluene did the trick, although the researchers were surprised that a hydrocarbon solvent readily dissolved this charged molecule.

Second, they identified a negatively charged ion, called an anion, to pair with the cation. This anion doesn't react with the silicon cation, but prevents it from reacting with other chemicals. It plays a protective role because its negative charge is diffuse, spreading widely over its large molecular structure, which contains 20 fluorine atoms, Lambert notes.

Using X-ray crystallography, the researchers obtained an atomic-level picture of their quarry. They determined that it indeed holds the positive charge and is too far removed from the anion, at 4 angstroms, to be considered bonded.

Philip R. Boudjouk, a silicon chemist at North Dakota State University in Fargo, calls the work a "very important development in organometallic chemistry." He adds, "The silicon cation is quite a reactive species, so we should be able to do things with it that you normally can't do."

Boudjouk speculates that it may prove useful for catalyzing the formation of polymers. Already, the silicon cation appears to speed the process for making materials used in silicone products, such as lubricants and adhesives, Lambert reports.
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Title Annotation:new silicon cation developed
Publication:Science News
Article Type:Brief Article
Date:Jul 17, 1993
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