Question

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

Answer 1

Dear student,

Comparing CN^- and CO, we can say that CO forms comparatively more stable complexes with metals that CN^​-.
This can be explained on the basis of their π- back acceptor properties. Let us first understand how bonding takes place.

When the metal and ligand come close, first σ- bond is formed by the donation of electrons from the ligand to the metal d-orbitals. Since, these ligands are very effective σ- donors, they increase the electron density on the metal centre to a very large exetent. For the stabilisation of the complex, the metal centre then donates the excess electron density into the π^* antibonding orbitals of these ligands whch leads to the extra stabilisation of the M-L ( L= Ligand ) bond. This type of bonding is called synergic bonding.
This bonding is highly effective when the ligand has empty has empty π^* antibonding orbitals.

These antibonding orbitals are available in both CN^- and CO ligands but the effective of the back donation form the ligand to the antibonding orbital of CO is slightly higher than CN^- because of the comparable energies of the orbitals of metal and ligand orbitals.

This type of bonding leads to higher crystal field splitting in the complex, which can also be regarded as another reason for the stabilisation of CO complexes more than CN^- complexes.

So, in a nutshell, we can say that CO forms slightly more stable complexes than CN^- with the metal centres.

Regards