$$ \text { In }\left[\mathrm{Co}\left(\mathrm{NH}_3\right) 6\right]^{3+}, \mathrm{Co}^{3+} \text { ion is having } 3 \mathrm{~d}^6 \text { configuration. } $$

$$ \text { In presence of } \mathrm{NH}_3 \text { ligand, pairing of electrons takes place and it becomes diamagnetic complex ion. } $$

$$\therefore \quad\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}$$ is octahedral with $$\mathrm{d}^2 \mathrm{sp}^3$$ hybridisation and it is diamagnetic in nature. In case of [CoF6]3-, Co is in +3 oxidation state and it is having 3d6 configuration.

In presence of weak field $\mathrm{F}$ - ligand, pairing does not take place.

$$ \therefore \quad \ln \left[\mathrm{CoF}_6\right]^{3-}, \mathrm{Co}^{3+} \text { is } \mathrm{sp}^3 \mathrm{~d}^2 \text { hybridised with four unpaired electrons, so it is paramagnetic in nature. } $$