Delaware scientists create shortest ever metal to metal bond
Chemists from the University of Delaware, Newark, in collaboration with the University of Wisconsin-Madison, recently reported their preparation of a complex containing two chromium atoms connected by the shortest ever metal-metal bond, 1.8028 Å (0.l nm) long. The scientists, including Prof. Klaus H. Theopold and graduate student Kevin A. Kreisel, believe the compound contains a quintuple bond, i.e., five bonds between the two chromiums.
To make the complex, the scientists first reacted chromium chloride with a bis(2,6-diisopropylphenyl)diazadiene ligand, which has the structure Ar-N=C-C=N-Ar (Ar is a bulky aryl group). This intermediate dark-green compound was then reduced using KC8 (potassium metal in graphite) to give the unusual product. The compound crystallizes from ether as red/green plates. The quintuple bond between the metals is weakened by some mixing in from antibonding orbitals, reducing the overall bond order to around 4.28. The bond length is 0.026 Å shorter than the previous record, a chromium-chromium complex prepared in 1978 by F. Albert Cotton, which contained a quadruple bond 1.828 Å long.
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