Credit: © 2008 Wiley

Bonding is at the very heart of chemistry and understanding the complexities and breadth of bonds is of great interest to a variety of chemists. Extremely high-order bonds have been observed to exist between metal atoms: quadruple- and quintuple-bonded species, for example. Although quintuple bonds have been predicted theoretically for several metals, only chromium atoms have been observed experimentally to share five electron pairs in five bonding orbitals. Now, groups based in Taiwan and Germany have independently prepared dichromium(I) complexes with the shortest reported metal–metal bond distances.

Yi-Chou Tsai and Chia-Wu Hsu at the National Tsing Hua University, in collaboration with three other Taiwanese universities, have made two coordination compounds1 that boast quintuply bonded chromium atoms. The first contains two chromium atoms only 1.82 Å apart, surrounded by three amidinate ligands. Density functional theory calculations confirmed the fivefold bonding between the metal atoms. This can be reduced by potassium graphite (KC8) to give a second complex with an even shorter Cr–Cr distance: 1.74 Å. This is the first metal–metal bond shorter than 1.8 Å.

Meanwhile, Rhett Kempe and Awal Noor at the University of Bayreuth, working with Frank Wagner at the Max Planck Institute for Chemical Physics of Solids in Dresden, have reduced chromium(II) compounds2 to create a dichromium(I) complex with an intermetallic distance of 1.75 Å. Interestingly, this contains only two coordinating ligands, in this case amidopyridinato groups. Although this also has a formal bond order of five, calculations involving electron density and the electron localizability indicator suggested a bond order of only 4.2. Kempe and colleagues attribute this to the repulsive nature of the 4s–4s bond at such short distances.

Both groups' complexes contain dichromium stabilized by nitrogen ligands, as was the case for the diazadiene dichromium compound that was previously the record holder at 1.80 Å. Although it is clear that the ligands have a crucial role in creating the conditions for such short bonds to exist, how exactly they do so is less clear. For example, other amidinate dichromium complexes, similar to those made by Tsai and colleagues, have much longer Cr–Cr bond lengths, up to 2.61 Å. Kempe and colleagues have ruled out weak coordination as an explanation for the strong bonding between the chromium atoms, because their ligand is strongly bonded to the metal atoms.