Structural and Stereo Isomerism in Coordination Compounds

• Classify all types of isomerism possible in coordination compounds
• Identify ionisation, linkage, coordination, and hydrate isomers from formulas
• Draw and distinguish cis/trans (geometric) isomers in square planar and octahedral complexes
• Predict fac and mer isomers in octahedral MA₃B₃ complexes
• Explain optical isomerism in [Co(en)₃]³⁺ and related chelate complexes
• Determine which isomers are chiral using the mirror image / symmetry test

Level 1 — Foundations

Isomers: Compounds with the same molecular formula but different arrangement of atoms.

Two main categories:

1. Structural (Constitutional) Isomers — differ in which atoms/groups are bonded together or their connectivity
2. Stereo Isomers — same connectivity, differ in spatial arrangement; subdivided into geometric and optical

Level 2 — JEE Depth

Structural Isomers

1. Ionisation Isomers
Same molecular formula, but different ions in the coordination sphere vs outer sphere.
Test: dissolve in water — different ions in solution give different precipitates.

Examples:

• [Co(NH₃)₅Br]SO₄ → gives SO₄²⁻ in solution (precipitate with Ba²⁺)
• [Co(NH₃)₅(SO₄)]Br → gives Br⁻ in solution (precipitate with AgNO₃)

2. Linkage Isomers
Ambidentate ligand (can bond through two different donor atoms) bonded differently.

Examples:

• [Co(NH₃)₅(NO₂)]²⁺ → NO₂⁻ bonded through N (nitro) → yellow-orange
• [Co(NH₃)₅(ONO)]²⁺ → NO₂⁻ bonded through O (nitrito) → red
• Similarly: SCN⁻ can bond via S (thiocyanato) or N (isothiocyanato)
• CN⁻ can bond via C (cyano) or N (isocyano)

3. Coordination Isomers
When both cation and anion are complex ions; ligands can be redistributed between them.

Examples:

• [Cu(NH₃)₄][PtCl₄] ↔ [Pt(NH₃)₄][CuCl₄]
• [Co(NH₃)₆][Cr(CN)₆] ↔ [Cr(NH₃)₆][Co(CN)₆]

4. Solvate (Hydrate) Isomers
Different number of water molecules inside vs outside the coordination sphere.

Examples (all CrCl₃·6H₂O, same formula):

• [Cr(H₂O)₆]Cl₃ — violet; all 3 Cl⁻ precipitated by AgNO₃ immediately
• [Cr(H₂O)₅Cl]Cl₂·H₂O — grey-green; 2 Cl⁻ precipitated
• [Cr(H₂O)₄Cl₂]Cl·2H₂O — dark green; 1 Cl⁻ precipitated

Stereo Isomers

5. Geometric (cis-trans) Isomers

Square planar MA₂B₂:

• Cis: same ligands adjacent (90° apart)
• Trans: same ligands opposite (180° apart)
• Example: [Pt(NH₃)₂Cl₂] — cis (cisplatin, cancer drug) and trans (transplatin, inactive)

Octahedral MA₄B₂:

• Cis: two B ligands at 90° (adjacent)
• Trans: two B ligands at 180° (opposite)

Octahedral MA₃B₃:

• Facial (fac): three A ligands occupy one face of octahedron (all mutually cis)
• Meridional (mer): three A ligands in a plane that includes two trans positions

Note: MA₆, MA₅B do NOT show geometric isomerism (only one possible arrangement).

6. Optical Isomers (Enantiomers)
Non-superimposable mirror images; rotate plane-polarised light in opposite directions.
d-form (dextrorotatory, +), l-form (levorotatory, −)

Conditions for optical isomerism: molecule has no plane of symmetry (chiral).

[Co(en)₃]³⁺: Three bidentate en ligands around octahedral Co³⁺

• No plane of symmetry in either arrangement
• d and l (or Λ and Δ) forms are non-superimposable mirror images
• Both are optically active

[Co(NH₃)₆]³⁺: Six identical monodentate ligands

• Has multiple planes of symmetry
• NOT optically active (achiral)

[CoCl₂(en)₂]⁺:

• Cis isomer: no plane of symmetry → chiral → shows optical isomerism (d and l)
• Trans isomer: has a plane of symmetry → achiral → NOT optically active

Summary Table

JEE Traps

• Square planar complexes: geometric isomers YES, optical isomers generally NO (molecule is planar → has plane of symmetry)
• Fac isomer of MA₃B₃ is chiral (no plane of symmetry); mer isomer is achiral
• Optical activity requires BOTH non-superimposability AND no internal mirror plane (meso compounds are achiral despite stereocentres)
• Ionisation isomers: verify by counting ions in solution, not just by looking at formula

Example 1: Cis and Trans Isomers of [Pt(NH₃)₂Cl₂]

[Pt(NH₃)₂Cl₂] — square planar complex (Pt²⁺ is d⁸, dsp² hybridisation)

Cis isomer (cisplatin):
  NH₃   Cl
    \  /
     Pt
    /  \
  NH₃   Cl
  
  NH₃ ligands adjacent (90°); Cl ligands adjacent (90°)
  Dipole moment ≠ 0 (non-zero, cis dipoles add)
  More soluble in water

Trans isomer (transplatin):
  NH₃   Cl
    \  /
     Pt
    /  \
  Cl   NH₃
  
  NH₃ ligands opposite (180°); Cl ligands opposite (180°)
  Dipole moment = 0 (trans dipoles cancel)
  Less soluble

Medical use: CIS isomer (cisplatin) is used in cancer chemotherapy.
It cross-links DNA strands (both Cl leave as leaving groups, forming bifunctional adduct).
Trans isomer cannot cross-link DNA the same way → inactive against tumours.

Answer: Cis isomer has same ligands adjacent; trans has same ligands opposite.
Cisplatin (cis form) is the anti-cancer drug — the structural difference has life-saving consequences.

Example 2: Optical Isomerism in [Co(en)₃]³⁺ vs [Co(NH₃)₆]³⁺

Why does [Co(en)₃]³⁺ show optical isomerism but [Co(NH₃)₆]³⁺ does not?

[Co(NH₃)₆]³⁺:
  6 identical NH₃ ligands arranged octahedrally around Co³⁺
  The molecule has MANY planes of symmetry (multiple C₄, C₃ rotation axes)
  Mirror image IS superimposable on the original
  → Achiral → No optical isomerism

[Co(en)₃]³⁺:
  3 bidentate en (ethylenediamine) ligands, each forming a 5-membered chelate ring
  The three rings can be arranged in two ways:
  Δ (delta) form: rings arranged like right-handed propeller (clockwise)
  Λ (lambda) form: rings arranged like left-handed propeller (anticlockwise)
  
  These two forms are non-superimposable mirror images
  Neither has a plane of symmetry, centre of inversion, or improper rotation axis
  
  → Both forms are chiral → optical isomers (enantiomers)
  → Rotate plane-polarised light in opposite directions

Physical properties (same): melting point, solubility, density, colour
Chemical properties (same in achiral environment): reactivity with achiral reagents
Differ in: direction of rotation of plane-polarised light, reactivity with chiral molecules (enzymes, drugs)

Answer: [Co(en)₃]³⁺ is chiral due to helical arrangement of chelate rings (no symmetry element);
[Co(NH₃)₆]³⁺ has many planes of symmetry and is achiral → no optical isomerism.

• Q1: [Co(NH₃)₅Cl]SO₄ and [Co(NH₃)₅(SO₄)]Cl — name this type of isomerism. How would you distinguish them experimentally?
• Q2: [Cr(NH₃)₃(NO₂)₃] exists as fac and mer isomers. Which one is optically active?
• Q3: Does [Pt(NH₃)₂Cl₂] (square planar) show optical isomerism? Give a reason.
• Q4: How many geometric isomers are possible for [Co(NH₃)₄Cl₂]⁺?
• Stretch: Q5: [CoCl₂(en)₂]⁺ — draw all possible isomers. For each, state whether it shows optical isomerism and explain why. (Hint: there is a cis and a trans geometric isomer, and the cis itself has d and l forms)

NCERT Chapter 9 link: Section 9.5 "Isomerism in Coordination Compounds" — covers all four structural isomers and both stereo isomers with examples. NCERT explicitly discusses cisplatin as a cis isomer used in cancer treatment. The Λ/Δ notation for optical isomers in [Co(en)₃]³⁺ is mentioned.

Exam connections: JEE Mains: identify isomerism type from given formulas, count number of isomers for a given formula, identify chiral vs achiral. JEE Advanced: draw structures of fac/mer isomers, explain why cis-[CoCl₂(en)₂]⁺ is optically active but trans is not, application of cisplatin mechanism in organic/biological context.

Study strategy: Draw every complex type in this note as a 3D structure — isomerism is visual, not just verbal. Make a decision tree: Is the formula the same? → Isomers. Same connectivity? → Stereo. Same 3D arrangement? → Same compound. Practise drawing fac vs mer for octahedral MA₃B₃ until the distinction is automatic.

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