I know complex ion formation is one of the MCAT PS topics, although I'm not sure how much coordination chemistry you'll need to know there; maybe just know that there are ligands around the transition metal in a complex ion and the charge of that metal.
This is from TBR Chemistry Book: I think this should be sufficient. Know how it affects solubility also.
Complex lon Formation: Complex equilibrium occurs when two reactions are summed to form one overall reaction. This is possible when the product of one reaction is a reactant in a second reaction. A great example involves the addition of ammonium to salt buildup in sinks in order to remove the deposit. To check this, take a small whiff of a bathroom cleaner, and you will detect ammonia.
Because ammonia forms hydrogen bonds with water, the calcium-ammonia complex is more soluble than the calcium cation. By the addition of ammonia to solution, free calcium cation forms a complex ion (the result of Reaction 3.11 shifting in the forward direction), which reduces the amount of free calcium and forces the Reaction 3.10 to proceed in the forward direction. As mentioned prior, when reactions are added together, the equilibrium constant for the overall reaction is the product of the equilibrium constants of the component reactions.
Complex lons (Coordination complexes): Complex ions form when a ligand (lone pair donor or Lewis base) donates a pair of electrons to a central atom (Lewis acid), which is typically a metal, to form a coordinate covalent bond. This process is referred to as chelation, where the Lewis base is a chelating agent. A prime example is hemoglobin whose porphyrin ring serves as a polydentate ligand by chelating the central iron.