CSIR-NET Coordination Chemistry — Isomerism
Isomerism questions on coordination compounds are one of the most reliable marks-scorers in CSIR-NET Part C, precisely because the classification is systematic rather than a matter of chemical intuition. Get the classification tree straight — structural isomerism first, stereoisomerism second — and almost every question reduces to correctly counting or naming what is actually different between two formulas.
The classification tree
Stereoisomerism (same connectivity, different spatial arrangement): geometrical (cis/trans, fac/mer) · optical (Δ/Λ)
Worked example 1 — counting geometrical isomers
Q. How many geometrical isomers does the square-planar complex [Pt(NH₃)₂Cl₂] have, and how many does the octahedral complex [Co(NH₃)₃(NO₂)₃] have?
Square-planar [Pt(NH₃)₂Cl₂]: the two identical NH₃ ligands can be adjacent (cis) or opposite (trans) — 2 isomers. This is the textbook cis-platin / trans-platin pair.
Octahedral [MA₃B₃] type: only 2 isomers are possible here too, but by a different rule — facial (fac), where the three NH₃ ligands occupy one triangular face of the octahedron (and the three NO₂ ligands occupy the opposite face), and meridional (mer), where the three NH₃ ligands lie along a "belt" through the centre, with one NH₃ trans to another NH₃ and the third cis to both. Students very commonly overcount MA₃B₃ isomers by treating each arrangement of three positions as distinct — the correct count is always exactly 2 (fac and mer), never more.
Optical isomerism in tris-chelate octahedral complexes
A metal centre bound by three identical bidentate (chelating) ligands, [M(AA)₃], has no internal plane or centre of symmetry — the whole complex is chiral, existing as non-superimposable mirror-image forms even though no single atom is a classical "stereocentre." These are labelled Δ (delta, a right-handed propeller-like twist of the three chelate rings) and Λ (lambda, left-handed).
Worked example 2 — [Co(en)₃]³⁺
Q. Why is [Co(en)₃]³⁺ (en = ethylenediamine, a bidentate ligand) optically active, and what are its two forms called?
Three bidentate en ligands wrap around the octahedral Co(III) centre with an inherent propeller-like twist — the complex as a whole has no plane of symmetry to make it superimposable on its mirror image. It therefore exists as two non-superimposable enantiomers, Δ-[Co(en)₃]³⁺ and Λ-[Co(en)₃]³⁺. This exact complex was the compound Alfred Werner famously resolved into its two optically active forms in 1911, providing decisive evidence for his octahedral coordination theory — it remains the standard textbook example of chirality arising purely from ligand arrangement rather than from any individual stereocentre.
Worked example 3 — telling linkage and ionization isomers apart by a chemical test
Q. Distinguish (a) [Co(NH₃)₅(NO₂)]²⁺ from [Co(NH₃)₅(ONO)]²⁺, and (b) [Co(NH₃)₅Br]SO₄ from [Co(NH₃)₅SO₄]Br, in terms of isomer type and how you would tell them apart experimentally.
(a) Linkage isomers. NO₂⁻ is ambidentate — it can bind through nitrogen (nitro, [Co(NH₃)₅(NO₂)]²⁺, N-bonded, thermodynamically more stable, yellow-brown) or through oxygen (nitrito, [Co(NH₃)₅(ONO)]²⁺, O-bonded, less stable, red, and it slowly isomerises to the nitro form on standing or gentle heating). Infrared spectroscopy distinguishes them by the characteristic N–O stretching pattern, which differs between the two binding modes.
(b) Ionization isomers. These differ in which anion is coordinated and which is the free counter-ion. Test each with two separate reagents: [Co(NH₃)₅Br]SO₄ gives an immediate white precipitate with BaCl₂ (confirming free SO₄²⁻) but no precipitate with AgNO₃ (Br⁻ is coordinated, not free to react); conversely, [Co(NH₃)₅SO₄]Br gives an immediate pale-yellow precipitate with AgNO₃ (confirming free Br⁻) but no precipitate with BaCl₂ (SO₄²⁻ is coordinated). The precipitation test is the standard way this pair is distinguished in the lab, and in an exam answer.
Common mistakes that cost marks
- Confusing linkage isomerism with ionization isomerism. Linkage isomerism involves one ambidentate ligand binding through different atoms; ionization isomerism involves two different anions swapping between the coordination sphere and the counter-ion position — different phenomena entirely.
- Overcounting geometrical isomers of an MA₃B₃ octahedral complex. There are always exactly 2 (fac and mer) — never more, regardless of which specific ligands A and B are.
- Assuming every [M(AA)₃]-type octahedral complex is automatically optically active without checking symmetry. The general tris-chelate case is chiral, but a specific ligand set with its own internal symmetry (or an achiral chelate that introduces a mirror plane) can remove the chirality — always check the actual symmetry, not just the ligand count.
- Mixing up hydrate isomerism with ionization isomerism. Hydrate isomerism is a special case where water (rather than an anion) swaps between the coordination sphere and the lattice — classically shown by the three isomers of CrCl₃·6H₂O, which differ in how many water molecules are actually coordinated.
Isomerism type summary table
| Type | What differs | Example pair |
|---|---|---|
| Ionization | Which anion is coordinated vs. free counter-ion | [Co(NH₃)₅Br]SO₄ / [Co(NH₃)₅SO₄]Br |
| Linkage | Which atom of an ambidentate ligand binds the metal | [Co(NH₃)₅(NO₂)]²⁺ / [Co(NH₃)₅(ONO)]²⁺ |
| Geometrical | Relative position of ligands (cis/trans, fac/mer) | cis- / trans-[Pt(NH₃)₂Cl₂] |
| Optical | Non-superimposable mirror images | Δ- / Λ-[Co(en)₃]³⁺ |
Checking the overall charge or formula of a coordination compound before working through an isomerism question? The Molar Mass & Composition calculator confirms element counts for even complex coordination formulas.
Open the ABC Chemistry Calculator Suite →Preparing for CSIR-NET, IIT-JAM, GATE or CUET-PG? ABC Chemistry runs dedicated competitive-exam batches online across India — details at abcchemistry.in.