How can we classify ligands as strong and weak or intemediate?

You should learn the spectrochemical series to know which are weak field ligands and which are strong field ligands.

Weak field ligands:

I- , Br- , SCN- , Cl- , F- , OH- , NO2- , H2O.

These ligands doesn’t help in the pairing of unpaired electrons.

Strong field ligands:

NCS- , NCCH3 , NH3 , NO2- , CN- , NO , CO

These ligands help in pairing of the electrons.

Some times there are exceptional case when some weak field ligands acts as strong field ligand for example: [PtCl4]2-, CoF6

Here these ligands act as strong field ligands.

Regards

Arun (askIITians forum expert)

The strength of a ligand actually depends upon the metal atom to which it is coordinated to.
Consider an example: Cl^- is considered as a weak ligand. However, if you consider the complex [PtCl₄]^2- , here Cl acts as a strong field ligand and causes electron pairing in Pt²⁺. This is mainly because of the large size of Pt atom. For this reason,this complex is square planar and not tetrahedral.

But to an extent, the strength of a ligand can be found out relating to its electronegativity. If a species is highly electronegative it has less tendency to donate electrons and thus is a weak field ligand.But still it is not completely acceptable as seen in the above anomaly.

Electronic configuration of Pt is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s14f14 5d9

HOPE IT’s CLEAR TO YOU ...

Thanks!