Q104P - Chemistry: Molecular Nature Of Matter And Change

Question

A complex ${\mathbf{ML}}_{\mathbf{6}}^{\mathbf{2}\mathbf{+}}$is violet. The same metal forms a complex with another ligand, Q, that creates a weaker field. What colour ${\mathbf{MQ}}_{\mathbf{6}}^{\mathbf{2}\mathbf{+}}$ might be expected to show? Explain.

Step-by-Step Solution

Answer

The expected colour is Green or Blue.

Step 1: Given Information

The color of ${\mathrm{ML}}_6^{2+}$is violet.

Step 2: Concept Introduction

Every color has its complementary color: Red - Green Yellow - Violet Orange Blue When the same metal ion is in different compounds (different ligands), the color of the compound depends on the ligands. For example, when one replaces the strong-field ligand with a weak-field ligand the color changes to its complementary color. Spectrochemical sequence (from ligands with weak fields to those with strong fields):

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

The weaker the field is, the lower the splitting energy is $(∆)$, and the longer the absorbed wavelengths are

The stronger the field is, the higher the splitting energy is $(∆)$and the shorter the absorbed wavelengths are

Step 3: Solution

${\mathrm{ML}}_6^{2+}$ is violet, which means it absorbs yellow light. The Q is a weaker-field ligand, which means it absorbs light with low-sitting energy $\left(∆\right)$ and with longer wavelengths absorbed.

That means the possible absorbed light colors are red and orange. 

If the orange is absorbed, the observed color would be its complementary, blue. 

If the red is absorbed, the observed color would be its complementary color green.