Chemistry - Coordination Chemistry Question with Solution | TestHub

In metal carbonyls, bonding involves synergistic interaction where the CO ligand donates lone pair electrons to the metal via a $\sigma$-bond.

Simultaneously, the filled $d$-orbitals of the metal back-donate electron density into the empty antibonding ${\pi }^{\ast }$ molecular orbitals of CO.

Therefore, this interaction is specifically classified as $d\pi -{\pi }^{\ast }$ back bonding, not $d\pi -p\pi$ back bonding.

This back-donation decreases the bond order of CO (making option D true) and stabilizes low, zero, or negative oxidation states of the metal (making option A true).