Question

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

• A. $\left[Co\right(en{)}_3{]}^{3+},\left[Co\right(N{H}_3{)}_6{]}^{3+},\left[Co\right(H_{2}O{)}_6{]}^{3+}$
• B. $\left[Co\right(H_{2}O{)}_6{]}^{3+},\left[Co\right(en{)}_3{]}^{3+},\left[Co\right(N{H}_3{)}_6{]}^{3+}$
• C. $\left[Co\right(H_{2}O{)}_6{]}^{3+},\left[Co\right(N{H}_3{)}_6{]}^{3+},\left[Co\right(en{)}_3{]}^{3+}$
• D. $\left[Co\right(N{H}_3{)}_6{]}^{3+},\left[Co\right(en{)}_3{]}^{3+},\left[CO\right(H_{2}O{)}_6{]}^{3+}$

Answer 1

The correct option is A

$\left[Co\right(en{)}_3{]}^{3+},\left[Co\right(N{H}_3{)}_6{]}^{3+},\left[Co\right(H_{2}O{)}_6{]}^{3+}$

Key concept Wavelength $\left(\lambda \right)$ of absorption is inversely proportional to CFSE $\left({△}_{0}value\right)$ of ligands attached with the central metal ion
i.e. $\lambda \propto \frac{1}{{△}_0}$
According to spectrochemical series.
$I^{-}<B{r}^{-}<S^{2-}<SC{N}^{-}<C{l}^{-}<F^{-}<O{H}^{-}<C_2{O}_4^{2-}<O^{2-}<H_{2}O<NC{S}^{-}<N{H}_3<en<N{O}_2^{-}<C{N}^{-}$


The CFSE of ligands attached with $C{o}^{3+}$ ion is in the order en > $N{H}_3>H_{2}O$ (From spectrochemical series)
$\because$ Wavelength of absorbed light $\left(\lambda \right)\propto \frac{1}{{△}_0}$
$\therefore$ For ligand the order of wavelength of absorption in the visible region will be : en < $N{H}_3<H_{2}O$
or, $\left[Co\right(en{)}_3{]}^{3+}<\left[Co\right(N{H}_3{)}_6{]}^{3+}<\left[Co\right(H_{2}O{)}_6{]}^{3+}$