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 (0.2 nm) long. 1 Å is 1×10−10 meters or 1/10,000,000,000 meters. 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 chromium atoms.

In general, shorter bonds containing more electrons will be stronger. Although single, double and triple bonds occur in nature, quadruple and quintuple bonds have so far only been found in the laboratory. The first compound with a quintuple bond was reported in 2005.

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 crystallises 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 dichromium complex reported in 1978 by F. Albert Cotton and coworkers, which featured a quadruple bond 1.828 Å long.