Type of isomerism exhibited by the compounds $[\mathrm{Cr}(\mathrm{H}{}_2\mathrm{O}){}_6]\mathrm{Cl}{}_3$, $[\mathrm{Cr}(\mathrm{H}{}_2\mathrm{O}){}_5\mathrm{Cl}]\mathrm{Cl}{}_2\cdot \mathrm{H}{}_2\mathrm{O}$ and $[\mathrm{Cr}(\mathrm{H}{}_2\mathrm{O}){}_4\mathrm{Cl}{}_2]\mathrm{Cl}\cdot 2\mathrm{H}{}_2\mathrm{O}$, and the value of the coordination number (CN) of the central metal ion in all these compounds, respectively, is:

Solution & Explanation

\textbf{Answer:} (B) Solvate (hydrate) isomerism, CN = 6. \textbf{Solution:} The three compounds $[\mathrm{Cr}(\mathrm{H}{}_2\mathrm{O}){}_6]\mathrm{Cl}{}_3$, $[\mathrm{Cr}(\mathrm{H}{}_2\mathrm{O}){}_5\mathrm{Cl}]\mathrm{Cl}{}_2\cdot \mathrm{H}{}_2\mathrm{O}$ and $[\mathrm{Cr}(\mathrm{H}{}_2\mathrm{O}){}_4\mathrm{Cl}{}_2]\mathrm{Cl}\cdot 2\mathrm{H}{}_2\mathrm{O}$ have the same overall composition but differ in how many water molecules are inside the coordination sphere versus held in the lattice as water of crystallisation. This is the defining feature of \textbf{solvate (hydrate) isomerism}. In every one of these complexes the chromium centre is bonded to a total of six donor atoms (water + chloride filling the six coordination sites), so the \textbf{coordination number is 6}. Hence option (B).