JEE Main · 2020 · 08 Jan 2020 · Shift 1 · Coordination Compounds

Facial and meridional isomerism arises in octahedral complexes of the type $[\mathrm{M}{\mathrm{a}}_3{\mathrm{b}}_3]$, where three identical ligands of one kind and three of another surround the metal: in the fac isomer one set occupies a single triangular face, and in the mer isomer they lie around a meridian. The complex $[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_3(\mathrm{N}{\mathrm{O}}_2{)}_3]$ has exactly this $[\mathrm{M}{\mathrm{a}}_3{\mathrm{b}}_3]$ composition and so shows both forms, whereas the others either contain a bidentate ligand such as en or lack the three-and-three ligand split that fac-mer isomerism requires.