JEE Main · 2025 · 23 Jan 2025 · Shift 1 · Coordination Compounds

Facial–meridional isomerism occurs only in octahedral complexes of the general type $\mathrm{M}{\mathrm{a}}_3{\mathrm{b}}_3$, where three ligands of one kind can occupy either a triangular face (fac) or a meridian (mer) of the octahedron.

$\left[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_3\mathrm{C}{\mathrm{l}}_3\right]$ has three $\mathrm{N}{\mathrm{H}}_3$ and three $\mathrm{C}{\mathrm{l}}^{-}$ ligands, an $\mathrm{M}{\mathrm{a}}_3{\mathrm{b}}_3$ complex that shows fac and mer forms.

${\left[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_4\mathrm{C}{\mathrm{l}}_2\right]}^{+}$ ($\mathrm{M}{\mathrm{a}}_4{\mathrm{b}}_2$) shows cis–trans isomerism, ${\left[\mathrm{Co}(\mathrm{en}{)}_3\right]}^{3+}$ ($\mathrm{M}(\mathrm{AA}{)}_3$) shows optical isomerism, and ${\left[\mathrm{Co}(\mathrm{en}{)}_2\mathrm{C}{\mathrm{l}}_2\right]}^{+}$ ($\mathrm{M}(\mathrm{AA}{)}_2{\mathrm{b}}_2$) shows cis–trans isomerism.

Only $\left[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_3\mathrm{C}{\mathrm{l}}_3\right]$ exhibits facial–meridional isomerism.