On the basis of crystal field theory explain why Co(III) Forms paramagnetic octahedral complex with weak field ligands whereas it forms diamagnetic octahedral complex with strong field ligands.

Explanation

The electrons configuration of Co(III) ion is `[Ar]3d^(6)`

With weak field ligand e.g. `H_(2)O Delta_(0)` is quite small `(Delta_(0))ltP`. Therefore, two d-electrons will be `e_(g)` orbitals while four will remain in `t_(2g)` orbitals. With strong field ligand (eg. CO), `Delta_(0)` is quite larger `(Delta_(0))gtP`. Under the circumstances, no electron will occupy `e_(g)` orbital. All what remain in `t_(2g)` orbitals as shown below: