In both ${\left[\mathrm{\text{C}}\mathrm{\text{o}}{\left(\mathrm{\text{N}}{\mathrm{\text{H}}}_3\right)}_6\right]}^{3+}\mathrm{\text{a}}\mathrm{\text{n}}\mathrm{\text{d}}{\left[\mathrm{\text{C}}\mathrm{\text{o}}{\mathrm{\text{F}}}_6\right]}^{3+},\mathrm{\text{C}}\mathrm{\text{o}}$ is present as $\mathrm{\text{C}}{\mathrm{\text{o}}}^{3+}.$

Thus, the electronic configuration of Co is

Co=[Ar] $3{\text{d}}^7,4{\text{s}}^2$

Co³⁺=[Ar]$3{\text{d}}^6,4{\text{s}}^0$

In case of ${\left[\mathrm{\text{C}}\mathrm{\text{o}}{\left(\mathrm{\text{N}}{\mathrm{\text{H}}}_3\right)}_6\right]}^{3+},\mathrm{\text{N}}{\mathrm{\text{H}}}_3$ is a strong field ligand, so pairing of electrons in 3d-orbital takes place.

Co³⁺=[Ar]$3{\text{d}}^6,4{\text{s}}^0$




In ${\left[\mathrm{\text{C}}\mathrm{\text{o}}{\mathrm{\text{F}}}_6\right]}^{3+},\mathrm{\text{F}}$ is a weak field ligand, thus doesn't cause pairing. Hence,

Co³⁺=[Ar]$3{\text{d}}^6,4{\text{s}}^0$