CO is a stronger ligand than NH3 for many metals why?

According to Ligand field theory ; the  π-acceptor ligands are the stronger ligand and the π-donating ligand is a weak field ligand. here, we know that NH3 is a  π-donor in comparison to the CO which is a  π-acceptor ligand

 In case of CO ;

 It  not only contain lone pair of electrons but also have vacant π or π* -orbitals.  So, it works as π-acceptor and it forms a σ-bond with metal atoms by donating the lone electron pair and at the same time it also forms a π-bond  by accepting electrons from the filled metal orbitals
Hence, when CO approaches to an octahedral field, it has an unoccupied π-orbitals at energies greater than the metal dxy ,dxz , and dyz orbitals .

So the mixing of both occurs to form π and π* orbitals, where the  π* orbitals are localized on ligands and not on the metal so the nonbonding d-orbitals before reaction, now become  π orbitals which increases crystal field stabilizing energy Δo .

Now, if Δo is gighers so the CO will be called as strong field ligand while in case of π-donor ligand like NH3 , the process is reversed and the value of Δo becomes lower so they behave as weak ligand, because the π -orbitals are occupied in this case.

For diagrammatic representation:

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