The number of unpaired electrons in the complex ion[CoF₆]³⁻is (Atomic no. of Co=27)

In ${\left[{\mathrm{CoF}}_6\right]}^{3-},$ the oxidation state of Co is +3. Atomic number of Co is 27 and its electronic configuration is $\left[\mathrm{Ar}\right]3{\mathrm{d}}^{7}4{\mathrm{s}}^2.$ When CO becomes Co³⁺, its electronic configuration becomes $\left[\mathrm{Ar}\right]3{\mathrm{d}}^6.$

As Fluoride ion is a weak ligand, therefore no pairing of electrons takes place in 3d. Hence, there are four unpaired electrons in ${\left[{\mathrm{CoF}}_6\right]}^{3-}.$