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

With weak field ligands, `Delta_(0)ltP`, (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 ligands `Delta_(0)gtP` (unpairing energy), so pairing oocurs thus, the electronic configuration will be `t_(2g)^(6)e_(g)^(0)`. It has no unpaired electrons and is diamagnetic.