Coordination Isomers
Definition and meaning of Coordination Isomers in chemistry.
Coordination isomers are coordination compounds containing both a complex cation and a complex anion that share the same overall chemical composition but differ in how the ligands are distributed between the two metal centers.
In more detail
This type of structural (constitutional) isomerism can only occur in salts built from two different metal complexes, since swapping ligands between the cationic and anionic centers produces a distinct compound while keeping the total formula unchanged. Because each metal experiences a different ligand field in each isomer, coordination isomers often differ noticeably in color, magnetic behavior, and reactivity, even though their bulk elemental composition is identical. Coordination isomerism is distinct from ionization isomerism (which exchanges a ligand with a counter-ion) and from linkage isomerism (which involves an ambidentate ligand binding through a different atom).
Key facts
| Isomer Class | Structural (constitutional) isomerism |
|---|---|
| Requires | A salt with both a complex cation and a complex anion (different metals) |
| Example Pair | [Co(NH3)6][Cr(CN)6] / [Cr(NH3)6][Co(CN)6] |
| Field | Inorganic Chemistry |
[Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6] are coordination isomers: in the first, cobalt(III) is bound by six ammine ligands and chromium(III) by six cyanide ligands, while in the second the ligand assignment is reversed, though both share the same overall formula.
Frequently asked questions
How does coordination isomerism differ from ionization isomerism?
Coordination isomerism redistributes ligands between two different metal centers in a cation-anion complex pair, while ionization isomerism exchanges a ligand within a single complex ion for an external counter-ion, changing which species is inside versus outside the coordination sphere.
Why do coordination isomers have different properties despite the same formula?
Each metal center experiences a different set and arrangement of ligands in each isomer, which changes the crystal field splitting and bonding, leading to differences in color, magnetism, and chemical reactivity.