Account for the different magnetic behaviour of `[Co(CN_6)]^(3-) and [CoF_6]^(3-)`.

Explanation

In both the cases Co is present in + 3. O.S. i.e. Co (III). So electronic configuration of Co is `d^6` i.e., there are four unpaired electrons.

Now as we know `CN^(-)` -is a strong field ligand i.e electron pairing will take place. As a result number of unpaired electrons is zero. But on the other hand in case of `[CoF_6]^(3-), F^(-)` is weak field ligand so no electron .pairing will take place and number of unpaired electrons is four. Due to presence of unpaired electrons `[CoF_6]^(3-)` is paramagnetic. But `[Co(CN)_6]^(3-)` is diamagnetic due to absence of unpaired electrons.