Published online 15 August 2008 | Nature | doi:10.1038/news.2008.1042

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Photo finish for shortest metal-metal bond

In the ångström Olympics, competition for the chromium medal is fierce.

When in March this year Rhett Kempe submitted a paper for publication that reported the shortest ever bond between two atoms of metal in a stable molecule, he was confident of taking the record. In the compound made by Kempe and Awal Noor at Bayreuth University in Germany, and Frank Wagner from the Max Planck Institute for the Chemical Physics of Solids, Dresden, Germany, there were two chromium atoms separated by a teeny 1.75 ångströms (0.175 nanometres)1 — smashing the previous record, set last year, of 1.8 ångströms.

But by the time Kempe’s paper hit Angewandte Chemie International Edition’s website, another paper had appeared, submitted exactly one week after Kempe’s, reporting a chromium–chromium bond of just 1.74 ångströms2. “We only held the record for one week,” Kempe laments.

Eight years ago, wrestler Alexandre Kareline discovered second place wasn’t much fun.Associated Press

In terms of publication time, it could be argued he never held the record at all: the paper by Yi-Chou Tsai from the National Tsing Hua University in Taiwan, and his team made it through peer review a couple of weeks quicker, and was published online on 6 August. Kempe’s paper’s official publication date is 13 August. “We were so happy that we went down by this much,” says Kempe, “and we were so sad when we read [Tsai’s paper].”

Both groups report chromium compounds in which the two metal atoms are squeezed close so close by neighbouring groups that they form a quintuple bond – sharing five pairs of electrons.

Kempe’s chromiums are confined by two bulky ligands each of which contains two nitrogen atoms. Tsai’s molecule has the two chromiums squeezed by three different ligands, also attached to the metals by nitrogen atoms. Kempe says that he was surprised to see that the three-ligand system could produce a bond shorter than his. Binding ligands takes electrons, too, and so the more electrons are used for that purpose, the fewer the metal atoms have available for bonding to each other.

Shorter still and shorter

The fight for the shortest bond began in earnest last year when Klaus Theopold from the University of Delaware, Newark, and his colleagues stumbled across a compound with a chromium–chromium bond length of 1.8 ångströms3, breaking a record that had lasted for almost 30 years. That record was held by one of the world’s most formidable inorganic chemists, Al Cotton, at Texas A&M University in College Station4.

“It’s visceral. It’s about chemical bonds.”

Klaus Theopold
University of Delaware

“I had visions of holding the record for a while,” says Theopold. “Little did I know that this would be the beginning of a rapid series of papers reporting ever shorter bonds of this type.”

The range of metal-metal distances reported by Kempe and Tsai hasn’t been seriously considered before, says Philip Power, at the University of California, Davis, who in 2005 became the first person to make a stable compound with a quintuple bond between two metal atoms. “These experimental results show that we have much to learn about metal-metal bonding between transition metals,” says Power.

The bond lengths in these chromium-based molecules are nowhere near as short as the shortest element-element bond, which is 0.74 ångströms in hydrogen molecules. But chromium atoms are much bigger than hydrogen atoms, and their nuclei are swathed in great clouds of repulsive electrons.

Purely academic

A single chromium–chromium bond in a regular molecule might be as long as 3.4 ångströms. But when chromium atoms in a cold gas form brief liasons the distance between them can be as short as 1.68 ångströms. Because those cold atoms have no ligands to worry about, they can use all their electrons for bonding; so that distance probably represents the shortest that the metal–metal bond in a stable molecule can get.

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The quest for the shortest bond is purely academic, says Kempe. Although he says that these chromium molecules might find uses as catalysts for making polymers, like polyethylene, from organic molecules with carbon–carbon double bonds, a difference of a tenth of an ångström in the metal–metal bonds won't make any difference to their performance.

Theopold says that he hasn’t thought of a practical use for these compounds either, but that doesn’t put him off the work. “It’s visceral,” he says. “It is about chemical bonds. That is what chemists think about all the time, that is our business.”

Kempe is down, but not out. “We are not chasing a theoretical limit, we are chasing a practical limit,” he says, referring to the 1.68 ångströms seen in the gas-phase reactions. “Now we want to go to that limit.” 

  • References

    1. Noor, A., Wagner, F. R. & Kempe, R. Angew. Chem. Int. Edn Engl. doi: 10.1002/anie.200801160 (2008).
    2. Tsai, Y.-C. et al. Angew. Chem. Int. Edn Engl. doi: 10.1002/anie.200801286 (2008).
    3. Kreisel, K. A. et al. J. Am. Chem. Soc. 129, 14162 (2007). | Article | PubMed | ChemPort |
    4. Cotton, F. A., Koch, S. A. & Millar, M. Inorg. Chem. 17, 2084 (1978). | Article | ChemPort |
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