Correct increasing order for the wavelength of absorption in the visible regin for the complexes +3 is

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${[Co {(H_{2} O)}_{6}]}^{3 +}, {[Co {({NH}_{3})}_{6}]}^{3 +}, {[Co (en)_{3}]}^{3 +}$

${[Co {({NH}_{3})}_{6}]}^{3 +}, {[Co (en)_{3}]}^{3 +}, {[Co {(H_{2} O)}_{6}]}^{3 +}$

${[Co (en)_{3}]}^{3 +}, {[Co {({NH}_{3})}_{6}]}^{3 +}, {[Co {(H_{2} O)}_{6}]}^{3 +}$

${[Co {(H_{2} O)}_{6}]}^{3 +}, {[Co (en)_{3}]}^{3 +}, {[Co {({NH}_{3})}_{6}]}^{3 +}$

answer is B.

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Detailed Solution

The increasing order for the wavelengths of absorption in the visible region for the complexes of Co³⁺ is [Co(en)₃]³⁺<[Co(NH₃)₆]³⁺<[Co(H₂O)₆]3+.

The decreasing order of ligands strength is en > NH₃ >H₂O. Stronger ligand causes greater splitting between two energy levels.

The energy of light absorbed is more and the wavelength of light absorbed is less for the stronger ligand.

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