Q.Which kind of isomerism is possible for a complex [Co(NH₃)₄Br₂]Cl?

Step 1. [Co(NH₃)₄Br₂]Cl is an octahedral complex of the form [MA₄B₂] (4 NH₃ + 2 Br inside the coordination sphere, Cl⁻ as the outer counter ion).

Step 2. [MA₄B₂] octahedral complexes show cis/trans geometrical isomerism (the two Br ligands can sit adjacent or opposite each other) -- per Section 5.5.2's octahedral discussion.

Step 3. The free Cl⁻ counter ion can itself act as a ligand, and swapping it with one of the coordinated Br⁻ ligands gives the isomer [Co(NH₃)₄ClBr]Br -- exactly the ionisation-isomerism pattern from Section 5.5.1.3 (this chapter even lists [Co(NH₃)₄Br₂]Cl and [Co(NH₃)₄ClBr]Br as its own worked ionisation-isomer example). …

Check the octahedral ligand pattern [MA₄B₂] for cis/trans geometrical isomerism, and separately check whether the outer counter ion can exchange with an inne …

• Only spotting the geometrical isomerism from the [MA₄B₂] pattern and missing that this exact complex (with its exchangeable Cl⁻ counter ion) is the chapter's own named ionisation-isomer example. …