A halogen bond route to shorten the ultrashort sextuple bonds in Cr2 and Mo2

Sextuple bonded group 6 diatomics Cr-2 and Mo-2 possess ultrashort metal-metal bonds. Yet their bond dissociation energy is very low. The destabilising nature of sigma-bonds is responsible for this. Selective extraction of these sigma-electrons via a sigma-hole on a halogen bond donor shortens and strengthens the metal-metal bond. This study constitutes a hitherto unexplored application of halogen bonding and an example for the true violation of bond order-bond strength relation.