Sextuple bonded group 6 diatomics Cr2 and Mo2 possess ultrashort metal-metal bonds. Yet their bond dissociation energy is very low. The destabilising nature of σ-bonds is responsible for this. Selective extraction of these σ-electrons via a σ-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.