Ionization Isomers
Definition and meaning of Ionization Isomers in chemistry.
Ionization isomers are coordination compounds that share the exact same molecular formula but differ in which ion is bonded to the metal inside the coordination sphere and which one sits outside as a free counter-ion. Because the counter-ion dissociates in solution while the coordinated ion does not, the two isomers produce different ions when dissolved in water.
In more detail
Because the ligand bonded directly to the metal center affects crystal field splitting and geometry, ionization isomers often differ in color, conductivity, and reactivity even though their overall composition is identical. This makes them a practical illustration of how the inner coordination sphere, not just the overall formula, controls a complex's chemical behavior. Chemists distinguish them experimentally with selective precipitation tests, since only the free counter-ion reacts readily with an added reagent.
Key facts
| Field | Inorganic Chemistry |
|---|---|
| Example pair | [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br |
| Isomerism type | A subtype of structural (constitutional) isomerism, distinct from coordination isomerism |
| Detection method | Selective precipitation of the free counter-ion |
[Co(NH3)5Br]SO4 (pentaamminebromidocobalt(III) sulfate) and [Co(NH3)5SO4]Br (pentaamminesulfatocobalt(III) bromide) are ionization isomers. In the first, Br- is coordinated to cobalt and SO4^2- is the free counter-ion, so adding BaCl2 precipitates white BaSO4. In the second, SO4^2- is coordinated and Br- is free, so adding AgNO3 instead precipitates pale yellow AgBr.
Frequently asked questions
How are ionization isomers different from hydrate isomers?
Hydrate (hydration) isomers are a special case of ionization isomerism in which a water molecule and another ligand or counter-ion trade places, as in [Cr(H2O)6]Cl3 versus [Cr(H2O)5Cl]Cl2·H2O.
Why do ionization isomers behave differently in solution?
The ion inside the coordination sphere is covalently bonded to the metal and stays attached, while the ion outside is a free counter-ion that dissociates completely, so each isomer releases a different ion into solution.