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.

With weak field ligands: ${△}_0<P,$ (pairing energy) so, the electronic configuration of Co (III) will be $t_{2g}^4{e}_g^2$ i.e., it has 4 unpaired electrons and is paramagnetic.



With strong field ligands, ${△}_0>P,$ (pairing energy) so, pairing occurs thus, the electronic configuration will be at $t_{2g}^6{e}_g^0.$ It has no unpaired electrons and is diamagnetic.