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Question: The colour of the coordinatinos compounds depends on the crystal field splitting. What will be the c...

The colour of the coordinatinos compounds depends on the crystal field splitting. What will be the correct order of absorptions of wavelength of light in the visible region, for the complexes,

[Co(CN)6]3−\lbrack Co(CN)_{6}\rbrack^{3 -},[Co(H2O)6]3+\lbrack Co(H_{2}O)_{6}\rbrack^{3 +}?

A

[Co(CN3)6]3−>[Co(NH3)6]3+>[Co(H2O)6]3+\lbrack Co(CN_{3})_{6}\rbrack^{3 -} > \lbrack Co(NH_{3})_{6}\rbrack^{3 +} > \lbrack Co(H_{2}O)_{6}\rbrack^{3 +}

B

[Co(NH3)6]3+>[Co(H2O)6]3+>[Co(CN)6]3−\lbrack Co(NH_{3})_{6}\rbrack^{3 +} > \lbrack Co(H_{2}O)_{6}\rbrack^{3 +} > \lbrack Co(CN)_{6}\rbrack^{3 -}

C

[Co(H2O)6]3+>[Co(NH3)6]3+>[Co(CN)6]3−\lbrack Co(H_{2}O)_{6}\rbrack^{3 +} > \lbrack Co(NH_{3})_{6}\rbrack^{3 +} > \lbrack Co(CN)_{6}\rbrack^{3 -}

D

[Co(NH3)6]3+>[Co(CN)6]3−>[Co(H2O)6]3+\lbrack Co(NH_{3})_{6}\rbrack^{3 +} > \lbrack Co(CN)_{6}\rbrack^{3 -} > \lbrack Co(H_{2}O)_{6}\rbrack^{3 +}

Answer

[Co(H2O)6]3+>[Co(NH3)6]3+>[Co(CN)6]3−\lbrack Co(H_{2}O)_{6}\rbrack^{3 +} > \lbrack Co(NH_{3})_{6}\rbrack^{3 +} > \lbrack Co(CN)_{6}\rbrack^{3 -}

Explanation

Solution

The CFSE of the ligands is in the order :

H2O<NH3<CN−H_{2}O < NH_{3} < CN^{-}

Hence, excitation energies are in the order :

[Co(H2O)6]3+<[Co(NH3)6]3+<[Co(CN)6]3−\lbrack Co(H_{2}O)_{6}\rbrack^{3 +} < \lbrack Co(NH_{3})_{6}\rbrack^{3 +} < \lbrack Co(CN)_{6}\rbrack^{3 -}

For the relation E=hcλ⇒E∝1λE = \frac{hc}{\lambda} \Rightarrow E \propto \frac{1}{\lambda}

The order of absorption of wavelength of light in the visible region is :

[Co(H2O)6]3+>[Co(NH3)6]3+>[Co(CN)6]3−\lbrack Co(H_{2}O)_{6}\rbrack^{3 +} > \lbrack Co(NH_{3})_{6}\rbrack^{3 +} > \lbrack Co(CN)_{6}\rbrack^{3 -}