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, $[C o (N H_{3})_{6}]^{3 +}, [C o (C N)_{6}]^{3 -}, [C o (H_{2} O)_{6}]^{3 +}$
$[C o (N H_{3})_{6}]^{3 +}, [C o (C N)_{6}]^{3 -}, [C o (H_{2} O)_{6}]^{3 +}$
(a) $[C o (C N)_{6}]^{3 -} > [C o (N H_{3})_{6}]^{3 +} > [C o (H_{2} O)_{6}]^{3 +}$
(b) $[C o (N H_{3})_{6}]^{3 +} > [C o (H_{2} O)_{6}]^{3 +} > [C o (N H_{3})_{6}]^{3 +}$
(c) $[C o (H_{2} O)_{6}]^{3 +} > [C o (N H_{3})_{6}]^{3 +} > [C o (C N)_{6}]^{3 -}$
(d) $[C o (C N)_{6}]^{3 -} > [C o (N H_{3})_{6}]^{3 +} > [C o (H_{2} O)_{6}]^{3 +}$

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Solution

Answer (c). As we know that, strong field ligand split the five degenerate energy levels with energy separation than week field ligand, i.e., as the strength of ligand increases crystal field splitting energy increases.
Hence, $△ E = h v = \frac{h c}{λ}$
$\Rightarrow△ E \propto \frac{1}{λ} \Rightarrow λ \propto \frac{1}{△ E}$
As energy separation increases, the wavelength decreases.
Thus, the correct order is
$[C o (H_{2} O)_{6}]^{3 +} > [C o (N H_{3})_{6}]^{3 +} > [C o (C N)_{6}]^{3 -}$
Here, strength of ligand increases, $△ E$ increases, CFSE increases and $λ$ absorbed decreases.
Hence, the correct choice is (c).

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