Question

The color of the coordination compounds depends on the crystal field splitting. What will be the correct order of absorption of wavelength of light in the visible region, for the complexes,
${\left[Co{\left({NH}_3\right)}_6\right]}^{3+},{\left[Co\right(CN{)}_6]}^{3-},{\left[Co{\left(H_{2}O\right)}_6\right]}^{3+}$

• A. ${\left[Co\right(CN{)}_6]}^{3-}>{\left[Co{\left({NH}_3\right)}_6\right]}^{3+}>{\left[Co{\left(H_{2}O\right)}_6\right]}^{3+}$
• B. ${\left[Co{\left({NH}_3\right)}_6\right]}^{3+}>{\left[Co{\left(H_{3}O\right)}_6\right]}^{3+}>{\left[Co\right(CN{)}_6]}^{3-}$
• C. ${\left[Co{\left(H_{2}O\right)}_6\right]}^{3+}>{\left[Co{\left({NH}_3\right)}_6\right]}^{3+}>{\left[Co\right(CN{)}_6]}^{3-}$
• D. ${\left[Co{\left({NH}_3\right)}_6\right]}^{3+}>{\left[Co\right(CN{)}_6]}^{3-}>{\left[Co{\left(H_{2}O\right)}_6\right]}^{3+}$

Answer 1

The correct option is A ${\left[Co\right(CN{)}_6]}^{3-}>{\left[Co{\left({NH}_3\right)}_6\right]}^{3+}>{\left[Co{\left(H_{2}O\right)}_6\right]}^{3+}$

Stronger the ligand, stronger the crystal field stabilization energy, higher the wavelength of Absorption.
$C{N}^{-}>N{H}_3>H_{2}O$ [Strength of Ligands]
So, $\left[CO\right(CN{)}_6{]}^{3-}>\left[CO\right(N{H}_3{)}_6{]}^{3+}>\left[CO\right(H_{2}O{)}_6{]}^{3+}$ is the order of absorption of wavelength of light.