Condition for fac/mer isomerism

Fac–mer isomerism is shown by octahedral complexes of the type: $[MA_3B_3]$ where three identical ligands and another three identical ligands are present.

• fac: the three identical ligands occupy one face of the octahedron.
• mer: the three identical ligands lie in a meridional plane.

Check each option

Option A: $[CoCl_2(en)_2]$

• Here $en$ is a bidentate ligand.
• This is an octahedral complex of type $[M(en)_2Cl_2]$.
• Such complexes show cis/trans isomerism and possibly optical isomerism, but not fac/mer.

So, A is not correct.

Option B: $[Pt(NH_3)_2Cl_2]$

• Platinum(II) complexes are generally square planar.
• Square planar complexes can show cis/trans isomerism, not fac/mer.

So, B is not correct.

Option C: $[Co(NH_3)_3(NO_2)_3]$

• This is an octahedral complex of the type: $[MA_3B_3]$ with $A = NH_3$ and $B = NO_2$.
• Therefore it can show fac and mer isomerism.

So, C is correct.

Option D: $[Co(NH_3)_4Cl_2]^+$

• This is an octahedral complex of type: $[MA_4B_2]$
• Such complexes show cis/trans isomerism, not fac/mer.

So, D is not correct.

Final answer

The complex that can show fac- and mer-isomerism is: $\boxed{[Co(NH_3)_3(NO_2)_3]}$

Comparison with stored correct answer

Stored correct answer: C

Our derived answer: C

Hence, they agree.