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Physical Chemistry

Unique Multicenter Bond Confirmed

Spectroscopic evidence provides ultimate proof that a two-electron bond between four carbon atoms in a radical dimer really exists

by Stephen K. Ritter
May 6, 2013 | A version of this story appeared in Volume 91, Issue 18

This tetracyanoethylene dimer is held together by a two-electron bond (red lines) between four carbon atoms.
Structure of tetracyanoethylene dimer that is held by a two-electron bond between four carbon atoms.
This tetracyanoethylene dimer is held together by a two-electron bond (red lines) between four carbon atoms.

Chemical bonds typically involve two atoms sharing one or more pairs of electrons. But chemists keep finding molecules that defy that norm. For example, some electron-deficient molecules exhibit multicenter bonding with three atoms sharing two electrons. A dozen years ago, Joel S. Miller of the University of Utah and his colleagues discovered an unusual type of multicenter bonding: a molecule with a four-atom, two-electron bond. Miller’s team came across the anomaly when preparing magnetic materials from transition-metal complexes that include the radical anion tetracyanoethylene (TCNE). In the crystal structures they found that two TCNE units form a dimer (shown) in which the central carbons are separated by 2.9 Å. That distance exceeds conventional C–C bond lengths, which are in the range of 1.5 Å, but it is still close enough to suggest a bonding interaction. In addition, the dimer’s structural, spectroscopic, and magnetic properties point to a four-carbon, two-electron bond that is stronger than hydrogen bonds. “But there has still been a concern whether the bond really exists,” Miller says. His team now has definitive proof after elucidating telltale vibrational flexing of the bond by Raman spectroscopy (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201207813). Better understanding of this unique bonding could ultimately lead to new molecules with interesting magnetic and other properties, Miller notes.


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