Q103P - Chemistry: Molecular Nature Of Matter And Change

Question

Rank the following in order of decreasing $(∆)$ and energy of light absorbed: ${\left[\mathrm{Cr}{\left(\mathrm{en}\right)}_3\right]}^{\mathbf{3}\mathbf{+}}\mathbf{,}{\left[\mathrm{Cr}{\left(\mathrm{CN}\right)}_6\right]}^{\mathbf{3}\mathbf{-}}\mathbf{,}{\left[{\mathrm{CrCl}}_6\right]}^{\mathbf{3}\mathbf{-}}$

Step-by-Step Solution

Answer

The final answer in decreasing order is ${\left[\mathrm{Cr}{\left(\mathrm{CN}\right)}_6\right]}^{3-}>{\left[\mathrm{Cr}{\left(\mathrm{en}\right)}_3\right]}^{3+}>{\left[{\mathrm{CrCl}}_6\right]}^{3-}$

Step 1: Given Information

Given compounds are

 $$\begin{gathered} {\left[\mathrm{Cr}{\left(\mathrm{en}\right)}_3\right]}^{3+} \\ {\left[\mathrm{Cr}{\left(\mathrm{CN}\right)}_6\right]}^{3-} \\ {\left[{\mathrm{CrCl}}_6\right]}^{3-} \end{gathered}$$ 

Step 2: Concept Introduction

The stronger field the ligand is, the bigger the energy of light absorbed. Spectrochemical series (from weak-field to strong-field ligands): 

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

Step 3: Solution

We can now quickly formulate the answer to this problem using the spectrochemical series and the information above:

${\left[\mathrm{Cr}{\left(\mathrm{CN}\right)}_6\right]}^{3-}>{\left[\mathrm{Cr}{\left(\mathrm{en}\right)}_3\right]}^{3+}>{\left[{\mathrm{CrCl}}_6\right]}^{3-}$