Chemistry - Coordination Chemistry Question with Solution | TestHub

${\mathrm{Fe}}^{3+}$ and ${\mathrm{Co}}^{3+}$ have $3{\mathrm{d}}^5$ and $3{\mathrm{d}}^6$ electronic configurations respectively with 5 and 4 unpaired electrons. Since ${\mathrm{F}}^{-}$is weak ligand both form high spin outer orbital complexes. ${\left[{\mathrm{FeF}}_6\right]}^{3-}$ is colourless due to d-d transitions are forbidden but ${\left[{\mathrm{CoF}}_6\right]}^{3-}$ is blue coloured since the $6^{\text{th }}$ electron occupying $t_{2g}$ orbitals can be excited. As the number of unpaired electrons in ${\left[{\mathrm{CoF}}_6\right]}^{3-}$ are less than in ${\left[{\mathrm{FeF}}_6\right]}^{3-}$ the magnetic moment of ${\left[{\mathrm{CoF}}_6\right]}^{3-}$ is less than that of ${\left[{\mathrm{FeF}}_6\right]}^{3-}$.