Facial ($$fac$$) and Meridional ($$mer$$) Isomerism

Isomeric Geometry Condition:

$$\text{Facial (fac) Isomer: } 3 \text{ donor atoms of same ligand occupy vertices of a single octahedral face } (90^\circ\text{--}90^\circ\text{--}90^\circ \text{ cis positions})$$

$$\text{Meridional (mer) Isomer: } 3 \text{ donor atoms of same ligand lie along a meridian / plane passing through the metal } (90^\circ\text{--}90^\circ\text{--}180^\circ)$$

Concept:

Octahedral complexes of the formula $$[Ma_3b_3]$$ (e.g., $$[\text{Co}(\text{NH}_3)_3(\text{NO}_2)_3]$$) exhibit geometrical isomerism called $$fac$$-$$mer$$ isomerism rather than $$cis$$-$$trans$$. Both $$fac$$ and $$mer$$ isomers contain a plane of symmetry (POS) and are therefore optically inactive (achiral).

Notation:

• $$a, b$$ = Unidentate ligands
• $$fac$$ = Facial alignment (all identical ligands adjacent)
• $$mer$$ = Meridional alignment (ligands encircle the central atom in a plane)