Question

Correct increasing order for the wavelength of absorption in the visible region for the complexes of $C{o}^{3+}$ is :

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• A. $\left[Co\right(CN{)}_6{]}^{3+},\left[Co\right(N{H}_3{)}_5\left(H_{2}O\right){]}^{3+},\left[Co\right(N{H}_3{)}_{5}Cl{]}^{2+},\left[Co\right(N{H}_3{)}_6{]}^{3+}$
• B. $\left[Co\right(N{H}_3{)}_{5}Cl{]}^{2+},\left[Co\right(N{H}_3{)}_5\left(H_{2}O\right){]}^{3+},\left[Co\right(N{H}_3{)}_6{]}^{3+},\left[Co\right(CN{)}_6{]}^{3-}$
• C. $\left[Co\right(CN{)}_6{]}^{3-},\left[Co\right(N{H}_3{)}_6{]}^{3+},\left[Co\right(N{H}_3{)}_5\left(H_{2}O\right){]}^{3+},\left[Co\right(N{H}_3{)}_{5}Cl{]}^{2+}$
• D. $\left[Co\right(N{H}_3{)}_6{]}^{3+},\left[Co\right(CN{)}_6{]}^{3-},\left[Co\right(N{H}_3{)}_5\left(H_{2}O\right){]}^{3+},\left[Co\right(N{H}_3{)}_{5}Cl{]}^{2+}$

Answer 1

The correct option is C $\left[Co\right(CN{)}_6{]}^{3-},\left[Co\right(N{H}_3{)}_6{]}^{3+},\left[Co\right(N{H}_3{)}_5\left(H_{2}O\right){]}^{3+},\left[Co\right(N{H}_3{)}_{5}Cl{]}^{2+}$

Strength of ligands attached with $C{o}^{3+}$ ion is in the order of $C{N}^{-}>N{H}_3>H_{2}O>C{l}^{-}$.
So, order of crystal field splitting energy (Value of ${\Delta }_o$) will be in the same order.
increasing order for the wavelength of absorption in the visible region for the complexes of $C{o}^{3+}$ is
$\left[Co\right(CN{)}_6{]}^{3-}<\left[Co\right(N{H}_3{)}_6{]}^{3+}<\left[Co\right(N{H}_3{)}_5\left(H_{2}O\right){]}^{3+}<\left[Co\right(N{H}_3{)}_{5}Cl{]}^{2+}$

$\because$ Wave length absorbing light $\propto \frac{1}{{\Delta }_o}$

Hence, option (a) is correct.