CHEMICAL BONDING

INORGANIC ANTIAROMATICITY
Al₄^4– is first reported all-metal system with 4n electrons

STEVE RITTER

Aromatic compounds--stabilized by 4n + 2 electrons--were once thought to be purely the domain of organic chemistry. But in the past few years, chemists have discovered several aromatic all-metal cluster systems. Now, the concept of antiaromaticity--destabilization observed in cyclic systems with 4n electrons--is following suit.

	ANTITRADITION Wang (left) and Boldyrev teamed up to make the antiaromatic Al44– anion, which is stabilized by three lithium cations (Li = red, Al = blue). IMAGE AND PHOTO COURTESY OF A. BOLDYREV, L-S. WANG

Chemistry professor Alexander I. Boldyrev of Utah State University and physics professor Lai-Sheng Wang of Washington State University's Tri-Cities campus and Pacific Northwest National Lab and their coworkers have made Li₃Al₄^– in which the Al₄^4– anion has four electrons [Science, 300, 622 (2003)].

The work is an extension of research that Boldyrev and Wang have carried out on all-metal aromatics, such as NaAl₄^– in which the Al₄^2– anion contains two electrons (C&EN, Sept. 24, 2001, page 39). They reasoned that if the Al₄^4– anion with two additional electrons could be made, it would be antiaromatic.

Li₃Al₄^– was prepared by laser vaporization of a LiAl alloy and separated from other Li_xAl_y^–species using time-of-flight mass spectrometry. It was characterized by photoelectron spectroscopy and ab initio calculations to determine the optimized structure and molecular orbital pictures, verifying the presence of the four electrons. The bonding patterns and the rectangular shape of the Al₄^4– unit are analogous to cyclobutadiene, which is the quintessential antiaromatic organic compound, the researchers note.

Further advancement of aromaticity and antiaromaticity into metal territory will be valuable for understanding the properties of metal clusters, bulk metals, and alloys, the researchers believe. Making Al₄^4– on a large scale is an interesting idea but might not be possible, Wang adds, although heavier group 13 analogs could be easier to make. Bimetallic compounds exhibiting antiaromatic structural units may already exist, he says.

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