Gold for the Shortest Bond?

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Science  29 Aug 2008:
Vol. 321, Issue 5893, pp. 1134
DOI: 10.1126/science.321.5893.1134d

As the Olympic Games come to a close, it's worth pointing out that chemists, like athletes, enjoy keeping records. What's the shortest bond? The longest? The weakest? The strongest? In the realm of metals, it turns out that chromium (Cr) has a special distinction. It has just enough electrons that when two Cr atoms come together in the gas phase, they can join in a sextuple bond. Unfortunately, making a compound you can handle in solution requires adding ligands, which bring with them extra electrons that lower the bond order to quintuple at most. After realization of a stable quintuply bonded Cr compound, the question shifted to how short the bond could be, and how equitably the five pairs of electrons were really being shared. Tsai et al. have now succeeded in pushing the Cr centers a little closer together, creating a Cr2 anion protected by three bidentate amidinate ligands that x-ray crystallography revealed to have a central bond length of just under 1.74 °. At nearly the same time, Noor et al. prepared a neutral Cr2 complex, similarly flanked by bidentate nitrogen ligands (in this case, two amidopyridines), with a bond length of just below 1.75 °. For comparison, the gas-phase sextuple comes in at 1.68 Å. — JSY

Angew. Chem. Int. Ed. 47, 10.1002/anie.200801286; 10.1002/anie.200801160 (2008).

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