Homework Statement

A carbonyl complex of Iridium (electronic configuration- [Xe] 4f14 5d7
6s2
) with the formula
IrCl(CO)(PPh3)2 is known as Vaska’s complex. Which of the following ligand substitution/s will
decrease the triple bond character of CO in Vaska’s complex

Relevant Equations

none

My attempt: If the triple bond character of CO is to reduce, then the extent of synergic bonding(backbonding) should increase. so with that logic, the groups to be introduced newly must have an electron donating nature. However, this is a single correct question and I think it can be both (a) and (b).
answer is given as (a). can somebody explain?

Unless Ir is special for this, any option will change the bond properties of the CO ligand, even if the change is small. To convince me otherwise, I'd need to see IR spectra corresponding to option b, c and d as well as the parent IrCl(CO)(PPh₃)₂.

Now it does specify a decrease in triple bond character, which is interpretively equivalent to lengthening of the CO bond, while the Ir-C bond shortens.

The effect is known as π-backbonding (what you call 'synergic bonding'), where π electrons from the metal are donated to the antibonding orbitals of the ligand. For this to be significant, the ligand must be a (strong) π-acceptor, but the metal must also have electrons available to donate. Indeed, the effect is pronounced in low-oxidation state metals, which Ir(I) is.

We know that CO is a good π-acceptor on par with PPh₃. Therefore, substituting a PPh₃ ligand for a worse π-acceptor would be expected to change the π-backbonding behavior in the complex, and (a) becomes the intuitive answer and you got that part.

You also mentioned (b), in which a Cl^-, a weak field ligand, is substituted by CH₃^-, which is a strong field ligand, but it does not have good π-acceptor properties (you can rationalize this with an MO diagram of CH₄
perhaps).