Ionization isomers are a type of coordination isomerism where the same molecular formula gives different ions in solution due to the exchange of ligands between the coordination sphere and the counter ions.

The given complex is $\left[\mathrm{Cr}\right(H_{2}O{)}_4\mathrm{Cl}\left(\mathrm{NO}{)}_2\right)]\mathrm{Cl}$. Here, the coordination sphere is $\left[\mathrm{Cr}\right(H_{2}O{)}_4\mathrm{Cl}\left(\mathrm{NO}{)}_2\right)]$^$+$ and the counter ion is ${\mathrm{Cl}}^{-}$. The nitrito ligand ($\mathrm{NO}{}_2^{-}$) is coordinated through nitrogen (nitro form), but it can also coordinate through oxygen (nitrito form), though that is linkage isomerism, not ionization isomerism.

For ionization isomerism, we exchange a ligand from the coordination sphere with a counter ion. In this case, the chloride ion (${\mathrm{Cl}}^{-}$) outside can exchange with the nitrito ligand ($\mathrm{NO}{}_2^{-}$) inside the coordination sphere. So, the ionization isomer should have the nitrito ligand as the counter ion and chloride inside the coordination sphere.

Thus, the ionization isomer is $\left[\mathrm{Cr}\right(H_{2}O{)}_4{\mathrm{Cl}}_2\left)\right]\mathrm{NO}{}_2$, which corresponds to the option: $\left[\mathrm{Cr}\right(H_{2}O{)}_4{\mathrm{Cl}}_2\left)\right]\left(\mathrm{NO}{}_2\right)$.

Let's verify the ions: This complex will give $\left[\mathrm{Cr}\right(H_{2}O{)}_4{\mathrm{Cl}}_2\left)\right]$^$+$ and $\mathrm{NO}{}_2^{-}$ in solution, while the original gives $\left[\mathrm{Cr}\right(H_{2}O{)}_4\mathrm{Cl}\left(\mathrm{NO}{)}_2\right)]$^$+$ and ${\mathrm{Cl}}^{-}$. The ions are different, confirming ionization isomerism.

Other options are incorrect:

• The first option has the same ions as original, just written differently.
• The third option is a linkage isomer (ONO for nitrito).
• The fourth option has water of hydration, not an ionization isomer.

Related Topics

Isomerism in Coordination Compounds: Coordination compounds exhibit various types of isomerism including structural isomerism (which includes ionization, coordination, linkage, and hydrate isomerism) and stereoisomerism (geometrical and optical). Ionization isomers arise when the counter ion exchanges with a ligand within the coordination sphere, resulting in different ionic species in solution.

Formulae

For a complex $\left[M\right(A{)}_n\left(B{)}_m\right)]X$, an ionization isomer is $\left[M\right(A{)}_{n-1}\left(B{)}_{m}X\right)]A$, where A and B are ligands, and X is the counter ion, provided A and X are similar in charge and type.