Electronic configuration of $\mathrm{Co}^{3+}$ $Image$

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

In presence of $\mathrm{NH}_3$ ligand $Image$

$\therefore \quad\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}$ is octahedral with $\mathrm{d}^2 \mathrm{sp}^3$ hybridisation and it is diamagnetic in nature.

In case of $\left[\mathrm{CoF}_6\right]^{3-}, \mathrm{Co}$ is in +3 oxidation state and it is having $3 \mathrm{~d}^6$ configuration.

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

In presence of $\mathrm{F}^{-}$ligands $Image$

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