Indicate the complex/complex ion which did not show any geometrical isomerism:

We need to identify the coordination complex that does not show geometrical isomerism among the given options.

Geometrical isomerism in octahedral coordination complexes depends on the spatial arrangement of ligands around the central metal atom.

Let us analyze each option based on its general formula:

Option A: The complex $$[Co(NH_3)_4Cl_2]^+$$ has the general formula $$[Ma_4b_2]$$. Octahedral complexes of this type exist in distinct cis and trans configurations and therefore exhibit geometrical isomerism.

Option B: The complex $$[Co(NH_3)_3(NO_2)_3]$$ has the general formula $$[Ma_3b_3]$$. Octahedral complexes of this type exhibit facial and meridional isomers, which represent a form of geometrical isomerism.

Option C: The complex $$[Co(CN)_5(NC)]^{3-}$$ has the general formula $$[Ma_5b]$$. In a regular octahedral geometry, all positions occupied by the five identical ligands are equivalent relative to the single unique ligand. Consequently, no alternative spatial arrangement is possible, meaning this complex cannot show geometrical isomerism.

Option D: The complex $$[CoCl_2(en)_2]$$ has the general formula $$[M(AA)_2b_2]$$, where $$en$$ is a symmetric bidentate ligand. Octahedral complexes of this type exhibit cis and trans geometrical isomers.

Therefore, the complex ion that does not show geometrical isomerism is $$[Co(CN)_5(NC)]^{3-}$$.

Hence the correct option is C.