Q108CP - Chemistry: Molecular Nature Of Matter And Change

Question

Three of the complex ions that are formed by ${\mathbf{Co}}^{\mathbf{3}\mathbf{+}}$ are ${\mathbf{\left[}\mathbf{Co}{\left(H_{2}O\right)}_{\mathbf{6}}\mathbf{\right]}}^{\mathbf{3}\mathbf{+}}\mathbf{,}{\mathbf{\left[}\mathbf{Co}{\left({\mathrm{NH}}_3\right)}_{\mathbf{6}}\mathbf{\right]}}^{\mathbf{3}\mathbf{+}}\mathbf{,}\mathbf{ }\mathbf{and}\mathbf{ }{\mathbf{\left[}{\mathbf{CoF}}_{\mathbf{6}}\mathbf{\right]}}^{\mathbf{3}\mathbf{-}}$. These ions have the observed colours (listed in arbitrary order) yellow-orange, green, and blue. Match each complex with its colour. Explain.

Step-by-Step Solution

Answer

The observed color of ${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$ is yellow-orange.

The observed color of ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}$ is blue.

The observed color of ${\left[{\mathrm{CoF}}_6\right]}^{3-}$ is Green.

Step 1: Given data

Given compounds are ${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+},{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}, \mathrm{and} {\left[{\mathrm{CoF}}_6\right]}^{3-}$

Arbitrary colors are

Yellow-orange, Blue, and Green.

Step 2: Color of the Compound

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 compounds (different ligands). For example, when we replace the strong-field ligand with a weak-field ligand, the color changes to its complementary color. Spectrochemical series (from weak-field to strong-field ligands): 

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

The weaker the field is, the lower the splitting energy and the longer the absorbed wavelength.

The stronger the field is, the higher the splitting energy and the shorter the absorbed wavelength

Step 3: Explanation

In ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}$,${\mathrm{NH}}_3$ is a strong-field ligand, which means it absorbs the light with higher splitting energy and shorter absorbed wavelengths among the given compounds; the color of the absorbed light is violet because it has a short range of wavelengths. The observed color in this compound is yellow-orange, the complementary color of violet.

In ${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$, ${\mathrm{H}}_2\mathrm{O}$ is a weaker-field ligand than ${\mathrm{NH}}_3$, which means it absorbs the light with lower splitting energy and with longer absorbed wavelengths; the color of the absorbed light is orange because it has a longer range of wavelengths than violet. The observed color in this compound is blue, the complementary color of orange. 

 

In${\left[{\mathrm{CoF}}_6\right]}^{3-}$, ${\mathrm{F}}^{-}$ is a weak-field ligand, which means it absorbs light with lower splitting energy and with the longest absorbed wavelengths among all the given compounds; the absorbed color in this compound is red because it has a long range of wavelengths. The observed color in this compound is green, the complementary color of red.