Question

The hexaaqua complex ${\left[\mathrm{Ni}{\left(H_{2}O\right)}_6\right]}^{\mathbf{2}\mathbf{+}}$is green, whereas thehexaammonia complex ${\mathbf{\left[}\mathbf{Ni}{\left({\mathrm{NH}}_3\right)}_{\mathbf{6}}\mathbf{\right]}}^{\mathbf{2}\mathbf{+}}$is violet. Explain

Short answer

${\left[\mathrm{Ni}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$ is green because it absorbs red light with lower splitting energy $\left(∆\right)$and with longer absorbed wavelengths because ${\mathrm{H}}_2\mathrm{O}$is a weak-field ligand.

${\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}$ is violet because it absorbs yellow light with higher splitting energy $\left(∆\right)$and shorter absorbed wavelengths because ${\mathrm{NH}}_3$is a strong-field ligand.

Step-by-step solution

Given information

Two Given complexes are${\left[\mathrm{Ni}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$and${\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}$

Ligand concept

The color of the complex compound depends on the strength of the ligand. The complementary color is emitted by the ligand. For example, if the ligand absorbs red, it emits its complementary color green.

${\mathbf{I}}^{\mathbf{-}}\mathbf{<}{\mathbf{Cl}}^{\mathbf{-}}\mathbf{<}{\mathbf{F}}^{\mathbf{-}}\mathbf{<}{\mathbf{OH}}^{\mathbf{-}}\mathbf{<}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{<}{\mathbf{SCN}}^{\mathbf{-}}\mathbf{<}{\mathbf{NH}}_{\mathbf{3}}\mathbf{<}\mathbf{en}\mathbf{<}{\mathbf{NO}}_{\mathbf{2}}^{\mathbf{-}}\mathbf{<}{\mathbf{CN}}^{\mathbf{-}}\mathbf{<}\mathbf{CO}$

Every color has its complementary color: Red - Green, Yellow - Violet, Orange - Blue. The compound's color depends on the ligands when we have the same metal ion in different complex compounds (different ligands). For example, when we replace a strong-field ligand with a weak-field ligand, the color changes to its complementary color. Spectrochemical series (from weak-field to strong-field ligands):

Explanation

The weaker the field is, the lower the splitting energy $\left(∆\right)$and the longer the absorbed wavelengths.

The stronger the field is, the higher the splitting energy is $\left(∆\right)$and the shorter the absorbed wavelengths.