The colour 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, ${[Co {({NH}_{3})}_{6}]}^{3 +}, {[Co (CN)_{6}]}^{3 -}, {[Co {(H_{2} O)}_{6}]}^{3 +}$

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${[Co {(H_{2} O)}_{6}]}^{3 +} > {[Co {({NH}_{3})}_{6}]}^{3 +} > {[Co {(CN)}_{6}]}^{3 -}$

${[Co {(CN)}_{6}]}^{3 -} > {[Co {({NH}_{3})}_{6}]}^{3 +} > {[Co {(H_{2} O)}_{6}]}^{3 +}$

${[Co {({NH}_{3})}_{6}]}^{3 +} > {[Co {(H_{2} O)}_{6}]}^{3 +} > {[Co {(CN)}_{6}]}^{3 -}$

answer is C.

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Detailed Solution

As energy separation increases, the wavelength decreases. Thus, the correct order is ${[Co {(H_{2} O)}_{6}]}^{3 +} > {[Co {({NH}_{3})}_{6}]}^{3 +} > {[Co (CN)_{6}]}^{3 -}$ Here, strength of. Ligand increases, $∆$ increases, CFSE increases and $λ$ absorbed decreases.