Explain geometrical isomerism in six-coordinated complex compounds or octahedral complexes.

(N/A) Geometrical isomerism in octahedral complexes arises due to the different possible arrangements of ligands around the central metal ion.
$(i)$ Complexes of type $[MX_4L_2]^{n \pm}$:
These complexes exhibit cis and trans isomers. In the cis-isomer,the two $L$ ligands are adjacent to each other,while in the trans-isomer,they are opposite to each other ($180$° apart).
Example: $[Co(NH_3)_4Cl_2]^+$.
$(ii)$ Complexes of type $[MX_3L_3]$:
These complexes exhibit facial $(fac)$ and meridional $(mer)$ isomerism.
- Facial $(fac)$ isomer: When the three identical ligands occupy the corners of one of the triangular faces of the octahedron.
- Meridional $(mer)$ isomer: When the three identical ligands occupy the positions around the meridian of the octahedron.
Example: $[Co(NH_3)_3(NO_2)_3]$.
$(iii)$ Complexes of type $[MX_2(LL)_2]^{n \pm}$:
These complexes also show cis and trans isomerism,where $LL$ represents a bidentate ligand and $X$ represents a monodentate ligand.