Indicate the complex/complex ion which did not show any geometrical isomerism.

(a) $\left[{\mathrm{CoCl}}_2{\left(\mathrm{en}\right)}_2\right]$shows cis-trans isomerism.

(b) ${\left[\mathrm{Co}\left(\mathrm{CN}{)}_5\right(\mathrm{NC})\right]}^{3-}$ is of MA₅B type complex, which will not show any isomerism.

(c) $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_3{\left({\mathrm{NO}}_2\right)}_3\right]$ shows facial and meridonial isomerism.

(d) ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{Cl}}_2\right]}^{+}$ shows geometrical (cis-trans) isomerism. In cis isomer, two Cl ligands are adjacent to each other. In trans isomer two Cl ligands are opposite to each other.

$\therefore$ Option (b) is correct.