Ionization isomerism is a form is isomerism applying to metal complexes. It relates to the way in which the ligands in these complexes bind to the metal centre.

Such complexes consist of a metal, usually a transition metal, surrounded by species known as ligands. In all complexes, a set of ligands binds to the metal centre through the donation of electrons to the metal's vacant d-orbitals. The ligands which bind in this way are known as inner-sphere ligands, because they are bound directly to the metal centre.

If the inner-sphere ligands balance the positive charge on the metal ion, no further ligands are required. However if the charge on these ligands (and many have no charge, e.g. water) is not enough to balance the charge on the metal ion, the remaining charge must be balanced by forming ionic bonds with another set of ligands - these are called outer-sphere ligands.

For example, in the compound cis-platin, [PtCl2(NH3)2] (an anti-cancer agent), the +2 charge on platinum is balanced by the two chloride ions with -1 charge, so no more ligands are needed (NH3, ammonia, is neutral). However in the compound [Fe(NH3)6]2+, the +2 charge on iron is not balanced by the inner-sphere ammonia ligands, so further ligands with a total charge of -2 are also required - perhaps two nitro ions, NO2-. These extra ligands do not bind directly to the metal centre but interact ionically with the complex, so they are outer-sphere.

In some complexes, it is possible for certain ligands to be either inner- or outer-sphere, swapping places with each other. Only charged ligands can do this, as only charged ligands can form part of the outer-sphere at all. This gives rise to the phenomenon of ionization isomerism.

For example in the compound Co(NH3)4Cl2(NO2), two ionization isomers are possible. In one, both chlorides bind directly to the cobalt centre (they are inner-sphere), and the nitro simply balances the charge (it is outer-sphere). In the other, the nitro and one chloride (inner-sphere) both bind to the cobalt, and the remaining chloride (outer-sphere) balances the charge.

These would be represented, respectively, as [Co(NH3)4Cl2]NO2 and [Co(NH3)4Cl(NO2)]Cl.

Ionization isomerism is so called because different ionization isomers give different ions in aqueous solution: the outer-sphere ligands are those which ionize in water.