The lengths of the metal-to-metal quintuple bonds and reactivity thereof

• Metal–metal quintuple bonded complexes as an interesting type of organometallic compounds.
• Quintuple bonded complexes contain only Cr and Mo across the periodic table.
• Quintuple bonded complexes have ultrashort metal-to-metal bonds.
• Bridging ligands are important players in stabilizing quintuple bonded complexes and determining the metal-to-metal quintuple bond lengths.
• The quintuple bonded complexes show rich and varied reaction chemistry.

This article reviews factors affecting the metal–metal bond lengths in the quintuple bonded complexes and reactivity thereof. The metal–metal bond length is the most intriguing metric in the multiple bonded dinuclear complexes. This is not only the case in the well-developed triple and quadruple bonding, but also in the newly-born quintuple bonding. Akin to the wide range of CrCr quadruple bond lengths found among the quadruple bonded complexes, the quintuple bonded dichromium complexes also show a large variation in the CrCr bond lengths, although only fifteen dichromium compounds have been characterized thus far. Donor atoms and steric pressure of the supporting ligands are two main factors affecting the CrCr quintuple bond lengths. The highly reducing nature and low coordination of the quintuple bonded bimetallic complexes render them reactive towards many small inorganic molecules and organic functionalities, and many interesting reactions, such as addition, complexation, cycloaddition and reductive cleavage, were discovered from them.

The low-coordinate quintuple bonded dinuclear group VI complexes show some parallel reactions with alkenes and alkynes.Figure optionsDownload as PowerPoint slide