Issue Date: January 14, 2008
Conflicting Inorganic Aromaticity
Aromaticity and antiaromaticity, once purely the domain of organic chemistry, has expanded in recent years to include organometallic complexes, all-metal clusters, and main-group compounds. In a new example, Angel Ugrinov and Ayusman Sen of Pennsylvania State University together with Shiv N. Khanna of Virginia Commonwealth University and coworkers report the synthesis and crystal structure of Te2As22-, the first four-membered ring anion made from a combination of group 15 and 16 elements (J. Am. Chem. Soc. 2008, 130, 782). The planar anion, sandwiched between two potassium 18-crown-6 units (shown), has alternating Te-As bond lengths and angles that give it a parallelogram geometry. The structure and molecular orbital analysis reveal a quite unusual electronic structure for the anion: δ-antiaromatic bonding coupled with π-aromatic bonding and a triplet electronic ground state, which leads to the distorted square shape and net aromatic character. While similar systems with this "conflicting aromaticity" have previously been observed in the gas phase, Te2As22- is the first example isolated as a solid.
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