Chemistry - Coordination Chemistry Question with Solution | TestHub

When central metal ion is same (here ${\mathrm{Ni}}^{2+}$ ), the absorption of colour depends on the ligand.

According to spectrochemical series, various ligands are as follows

$\mathrm{I}<{\mathrm{Br}}^{-}<{\mathrm{Cl}}^{-}<{\mathrm{NO}}_3^{-}<{\mathrm{F}}^{-}<{\mathrm{H}}_2\mathrm{O}$

$<{\mathrm{NH}}_3<{\mathrm{NO}}_2^{-}<{\mathrm{CN}}^{-}<\mathrm{CO}$

Thus, ${\mathrm{H}}_2\mathrm{O}$ is the weakest ligand among these, therefore the absorbed energy will be lowest in
${\left[\mathrm{Ni}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+},$ so it will absorb highest wavelength (red light).

$\mathrm{E}\propto \frac{1}{\mathrm{\lambda }}$

$\therefore$ The order of increasing wavelength is

${\left[\mathrm{Ni}{\left({\mathrm{NO}}_2\right)}_6\right]}^{4-}<{\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}<{\left[\mathrm{Ni}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$