Question

out of CN- and CO which ligand forms more stable complex with metal and why?

Answer 1

On referring the Spectrochemical Series, we find that CO is a stronger ligand than CN​-. 
Thus the metal complex formed with CN- will be more stable.

Answer 2

Hey dear,

● Answer -
CO (carbonyl)

● Explanation -
- The spectrochemical series is a list of ligands based on the strength of their interaction with metal ions.
- Spectrochemical series mentions CO to be a stronger ligand than CN-.
- This can be explained by presence of both s & p characters in metal-carbon bond.
- Also carbonyl group shows backbonding which strengthens the bond between metal and ligand.
- Therefore, CO ligand forms more stable complexes with metals.

Hope this helps you...