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

`Co^(3+)` ion has the electronic configuration `3d^(6)`. In the formation of complexes with weak-field ligands (i.e., `Delta_(o) lt P`), electronic configuration of `Co^(3+)` will be `t_(2g)^(4)e_(g)^(2)`. It has 4 unpaired electrons and so the complex is paramagnetic. But with strong - field ligands `(i.e., Delta_(o) gt P)`, configuration of `Co^(3+)` will be `t_(2g)^(6)e_(g)^(0)`. It has no unpaired electrons and so the complex is diamagnetic.