The type of isomerism shown by the complex $[\mathrm{CoCl}{}_2(\mathrm{en}){}_2]$ is:

Solution & Explanation

\textbf{Answer:} (C) Geometrical isomerism. \textbf{Solution:} The complex $[\mathrm{CoCl}{}_2(\mathrm{en}){}_2]$ is octahedral with two bidentate ethylenediamine (en) ligands and two chlorido ligands. The two $\mathrm{Cl}{}^{-}$ ligands can be arranged either adjacent to each other (\textbf{cis}, $90^{\circ }$ apart) or directly opposite (\textbf{trans}, $180^{\circ }$ apart). These two distinct spatial arrangements are \textbf{geometrical (cis/trans) isomers}. There is no change of donor atom (rules out linkage), no exchange of ions in/out of the coordination sphere (rules out ionization), and no swapping of ligands between two complex ions (rules out coordination). Hence the complex shows geometrical isomerism, option (C).