Explain the following : (3) `CO` is considered to be a stronger ligand than `NH_(3)` for some metals.

Explanation

In the formation of a coordination compound, ligand `CO` acts simultaneously as a `sigma`- base (because it donates a pair of electrons to the vacant orbital of the metal atom to form a `sigma`- bond) as well as a `pi-` acid, because it accepts a pair of electrons into its vacant antibonding `(pi^(**))` orbital form a filled d - orbital of metal to form a `pi-` bond. This type of `pi-` interaction increases magnitude of crystal field splitting energy, `Delta_(0)`. Hence, `CO` acts as a strong-field ligand.