The complex ion $[CoCl_2(en)_2]^+$ represents an octahedral complex of the type $[M(AA)_2a_2]$, where 'AA' is the symmetrical bidentate ligand ethane-1,2-diamine ($en$) and 'a' is the monodentate ligand chloride ($Cl^-$).

This specific type of complex exhibits Geometrical Isomerism consisting of two isomers :

1. Cis-isomer: The two chloride ions occupy adjacent positions (90°) relative to each other.
2. Trans-isomer: The two chloride ions occupy positions opposite to each other (180°).

Analysis of other options:

• Linkage isomerism requires ambidentate ligands (like $NO_2$ or $SCN$), which are not present here .
• Coordination isomerism occurs when both the cation and anion are complex ions, which is not the case .
• Ionization isomerism would require the exchange of a ligand with a counter ion outside the coordination sphere. Since only the complex ion is specified without a specific counter ion capable of exchange, this is not the primary isomerism defined for the ion itself .

Therefore, Geometrical isomerism is the correct answer.