Key idea: When do meridional (mer) and facial (fac) isomerism occur?

Meridional/facial isomerism is possible in octahedral complexes of the type: $[MA_3B_3]$ where there are three ligands of one kind and three of another kind.

• fac: the three identical ligands occupy one face of the octahedron.
• mer: the three identical ligands lie along a meridian; two are trans and one is cis to both.

Check each option:

Option A: $\mathrm{[Pt(NH_3)_2Cl_2]}$

• Platinum here is typically square planar.
• Square planar complexes show cis-trans isomerism, not mer/fac isomerism.
• Therefore, no meridional isomerism.

Option B: $\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]$

• This is an octahedral complex with two bidentate ligands $(\mathrm{en})$ and two chloride ligands.
• Such complexes can show cis-trans isomerism.
• It is not of the type $[MA_3B_3]$.
• Therefore, no meridional isomerism.

Option C: $\left[\mathrm{Co}(\mathrm{en})_{3}\right]$$

• This is an octahedral complex with three identical bidentate ligands.
• It can show optical isomerism ($\Delta$ and $\Lambda$ forms).
• Since all ligands are the same, fac-mer isomerism is not possible.
• Therefore, no meridional isomerism.

Option D: $\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{2}\right)_{3}\right]$

• This is an octahedral complex of the type: $[MA_3B_3]$
• Hence it can show fac and mer isomerism.
• Therefore, meridional isomerism is possible.

Conclusion

The complex which can exist as a meridional isomer is: $\boxed{\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{2}\right)_{3}\right]}$

Comparison with stored correct answer

Stored correct answer: D

Our derived answer: D

Hence, the answer agrees with the stored correct answer.