Isomerism

The essentials

Isomers have the same formula but different arrangements of atoms. In coordination compounds there are two main kinds: structural isomers, which have different bonds, and stereoisomers, which have the same bonds but different spatial arrangements.

Key points

• Linkage isomerism: an ambidentate ligand binds through different atoms, e.g. [Co(NH₃)₅(NO₂)]Cl₂ (nitro, yellow) and [Co(NH₃)₅(ONO)]Cl₂ (nitrito, red).
• Coordination isomerism: ligands interchange between cationic and anionic complexes, e.g. [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].
• Ionisation isomerism: a counter ion and a ligand swap places, e.g. [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄; they give different ions in solution.
• Solvate (hydrate) isomerism: water is inside or outside the sphere, e.g. [Cr(H₂O)₆]Cl₃ (violet) and [Cr(H₂O)₅Cl]Cl₂·H₂O (grey-green).
• Geometrical isomerism: square planar [MX₂L₂] (e.g. [Pt(NH₃)₂Cl₂]) and octahedral [MX₂L₄] (e.g. [Co(NH₃)₄Cl₂]⁺) show cis and trans forms. Octahedral [MA₃B₃] shows fac and mer forms. Tetrahedral complexes do not show geometrical isomerism.
• Optical isomerism: non-superimposable mirror images (enantiomers), common in octahedral complexes with didentate ligands, e.g. [Co(en)₃]³⁺. cis-[PtCl₂(en)₂]²⁺ is optically active, whereas the trans form is not because it has a plane of symmetry.

Exam tips

• To test for optical activity, look for a plane of symmetry; if there is one, the complex is not chiral.

Common mistakes

• Expecting geometrical isomers for tetrahedral MA₂B₂ complexes.