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How are coordination isomers formed by exchanging ligands between two complex ions?

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Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups. — coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B₁₂ is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

Coordination isomerism occurs in ionic coordination compounds that contain both complex cations and anions. These isomers are formed when the ligands associated with the two metal atoms are exchanged or redistributed between the two metal centers.1

This process can involve a complete swap of ligands between the two metal atoms. For example, the pair hexaamminecobalt(3+) hexacyanochromate(3−), $[C o (N H_{3})_{6}] [C r (C N)_{6}]$ , and hexaamminechromium(3+) hexacyanocobaltate(3−), $[C r (N H_{3})_{6}] [C o (C N)_{6}]$ , are coordination isomers.1

Alternatively, coordination isomers can be formed by a partial redistribution of ligands between the two metal atoms. A notable example is the pair consisting of tetraammineplatinum(2+) hexachloroplatinate(2−), $[P t (N H_{3})_{4}] [P t C l_{6}]$ , and tetraamminedichloroplatinum(2+) tetrachloroplatinate(2−), $[P t C l_{2} (N H_{3})_{4}] [P t C l_{4}]$ . In these cases, the chemical composition of the overall compound remains the same, but the distribution of ligands between the cationic and anionic complexes differs, resulting in distinct isomeric structures.1