Question

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.

Answer 1

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.