Question

What will be the correct order for the wavelengths of absorption in the visible region for the following:
$\left[Ni\right(N{O}_2{)}_6{]}^{4-},\left[Ni\right(N{H}_3{)}_6{]}^{2+},\left[Ni\right(H_{2}O{)}_6{]}^{2+}?$

Answer 1

In all the given complexes, the metal ion is $N{i}^{2+}$. The increasing field strengths of the ligands present as per electrochemical series are in the order:
$H_{2}O<N{H}_3<N{O}_2^{-}$
The energies absorbed for excitation will be in the order:
$\left[Ni\right(H_{2}O{)}_6{]}^{2+}<\left[Ni\right(N{H}_3{)}_6{]}^{2+}<\left[Ni\right(N{O}_2{)}_6{]}^{4-}$
As $E=\frac{hc}{\lambda },i.e.,E\alpha \frac{1}{\lambda };$ the wavelengths will be in the opposite order, that is as follows
$\left[Ni\right(H_{2}O{)}_6{]}^{2+}>\left[Ni\right(N{H}_3{)}_6{]}^{2+}>\left[Ni\right(N{O}_2{)}_6{]}^{4-}$