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X-ray crystallography hats revealed what appears to be the shortest metal-metal bond yet. The bond stands at just 0.18028 nm and is found in a dinuclear chromium complex, breaking the previous record set almost three decades ago. Klaus Theopold and Kevin Kreisel of the University of Delaware and their colleagues have taken to heart recent calculations that suggested that it might be possible to synthesise a dinuclear chromium complex with a Cr-Cr bond order higher than 4. Dichromium compounds have previously been made with quintuple bonds and are well-known in the history of multiple metal-metal bonding as containing some supershort quadruple bonds. By coincidence, the Delaware team has investigated chromium compounds containing coordinated diazadiene groups. "In the course of these investigations, we have discovered a dinuclear compound that exhibits a very short chromium-chromium distance. Indeed, in terms of Cotton's "formal shortness ratio" (FSR), this compound features the shortest chemical bond presently known," the researchers say. The researchers synthesised this novel material by the reduction of a diazadiene chromium halide complex potassium graphite. They then determined the crystal structure of the diamagnetic, bimetallic product and found it to consist of a Cr2 unit spanned by two diazadiene ligands. The astounding feature of this new compound is its very short Cr-Cr distance, which is just over 0.18 nanometres, [1.8028(9) Å]. The team suggests that the low formal oxidation state of the chromium atoms would hint at a Cr-Cr bond order above 4. Their density functional theory (DFT) calculations with Clark Landis of the University of Wisconsin, Madison, then confirmed the structure and accurately reproduced their measurements. "The calculations show high-order metal-metal bonding along with extensive delocalization over the diazadiene ligands," the researchers explain, hinting at a quintuple bond and a natural bond order of 4.28. The previous record was held by coordination pioneer F. Albert Cotton and his team at Texas A&M University, in 1978 published a 1.828 Å quadruple-bonded dichromium complex in the journal Inorganic Chemistry. However, old records broken simply become today's new record to break and it is unlikely that other chemists will not now rise to the challenge of finding a still shorter metal-metal bond. SpectroscopyNOW put the questioon of potential applications to Theopold. "Regarding utility or practical applications, I will have to pass. That's no guarantee that somebody else might not think of one, but I can't off the top of my head. This is basic science, and a serendipitous discovery to boot." Related links:
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Theopold, building shorter bridges in metal-metal complexes
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