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Physical Chemistry

The Chemical Bond

March 19, 2007 | A version of this story appeared in Volume 85, Issue 12

The renaissance in chemical bonding, an important pillar of chemistry, which focuses on new modes of bonding described in Stephen Ritter's article, emphasizes that "the chemical bond is not as simple as people think" (C&EN, Jan. 29, page 37).

Linus Pauling's oft-quoted statement, "There is a chemical bond between two atoms or groups of atoms in the case that the forces acting between them are such as to lead to an aggregate with sufficient stability to make it convenient for the chemist to consider it as an independent molecular species," conveys more of a philosophy of chemistry than a testable definition of a bond. As Roald Hoffmann states in Ritter's article, chemists are pushing the concept of the chemical bond to the limits and are certainly having "fun with the richness of bonding precisely because it's not clearly defined."

I would like to point out that, although thought to be well-understood, multicenter carbon-carbon bonding also contributes to the resurgence of interest in chemical bonding. The eclipsed, closed-shell [TCNE]22- (tetracyanoethylene) dimer with a 2.9-Å intradimer separation, for example, is best described as a two-electron, four-center (2e-/4c) carbon bond. This bond distance far exceeds that associated with conventional covalent C-C bonding (compare 1.54 Å in diamond) but falls short of approximately 3.5-Å van der Waals spacings. This 2e-/4c bonding model explains all of the structural, spectroscopic (electronic, vibrational, and nuclear magnetic), and magnetic properties observed for [TCNE]22-.

Examples of larger species such as the tert-butylphenalenyl radical with seven pairs of approximately 3.2-Å separated eclipsed carbon atoms also are best described via multicenter carbon bonding. While thought to be a mature area of understanding, the chemical bond is still on the forefront of contemporary research, and important new insight into chemical bonding will undoubtedly result.

Joel S. Miller
Salt Lake City



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