Isomerism in Coordination Compounds

Introduction

Isomers are compounds with the same molecular formula but different arrangements of atoms. In coordination compounds, this phenomenon becomes fascinating - two complexes can have identical formulas but completely different properties, colors, and even biological activities!

Types of Isomerism in Coordination Compounds

graph TD
    A[Isomerism] --> B[Structural Isomerism]
    A --> C[Stereoisomerism]

    B --> B1[Ionization]
    B --> B2[Hydrate/Solvate]
    B --> B3[Linkage]
    B --> B4[Coordination]
    B --> B5[Ligand]

    C --> C1[Geometrical]
    C --> C2[Optical]

    style B fill:#e74c3c
    style C fill:#3498db

Part 1: Structural Isomerism

Isomers with different bonding patterns - atoms are connected differently.

1. Ionization Isomerism

Different ions are produced when dissolved in water.

Occurs when: An anionic ligand can exchange with a counter ion outside the coordination sphere.

Classic Example

What’s happening:

• Isomer A: SO₄²⁻ is outside → precipitates with Ba²⁺
• Isomer B: Br⁻ is outside → precipitates with Ag⁺

More Examples

1. [Pt(NH₃)₄Cl₂]Br₂ and [Pt(NH₃)₄Br₂]Cl₂
2. [Co(NH₃)₅NO₃]SO₄ and [Co(NH₃)₅SO₄]NO₃
3. [Cr(H₂O)₆]Cl₃ and [Cr(H₂O)₅Cl]Cl₂·H₂O (also hydrate isomer)

2. Hydrate (Solvate) Isomerism

Water molecules are inside or outside the coordination sphere.

The Famous Chromium Chloride Isomers

What’s happening:

• Water inside = strongly bound, won’t evaporate easily
• Water outside = weakly held, evaporates readily

Another Example

[Co(H₂O)₆]Br₃ vs [Co(H₂O)₅Br]Br₂·H₂O vs [Co(H₂O)₄Br₂]Br·2H₂O

3. Linkage Isomerism

Occurs with ambidentate ligands - ligands that can coordinate through different atoms.

Common Ambidentate Ligands

Classic Example: Nitro vs Nitrito

Nitro Isomer (bonded through N):

[Co(NH₃)₅-NO₂]²⁺

• Yellow
• M-N bond
• More stable

Nitrito Isomer (bonded through O):

[Co(NH₃)₅-ONO]²⁺

• Red
• M-O bond
• Less stable, slowly converts to nitro form

Thiocyanate Example

4. Coordination Isomerism

Occurs in compounds with both cationic and anionic complexes - ligands exchange between the two metal centers.

Examples

Isomer Pair 1:

• [Co(NH₃)₆][Cr(CN)₆]
• [Cr(NH₃)₆][Co(CN)₆]

Isomer Pair 2:

• [Cu(NH₃)₄][PtCl₄]
• [Pt(NH₃)₄][CuCl₄]

Isomer Pair 3:

• [Co(NH₃)₆][Cr(C₂O₄)₃]
• [Cr(NH₃)₆][Co(C₂O₄)₃]

5. Ligand Isomerism

Same molecular formula but different ligands.

Example:

• [Co(NH₂CH₂CH₂NH₂)₃]³⁺ (three ethylenediamine)
• [Co(NH₂CH₂CH₂CH₂NH₂)₂NH₃)₂]³⁺ (two propylenediamine + two ammonia)

This is less common and less important for JEE.

Part 2: Stereoisomerism

Same bonding pattern but different spatial arrangements.

Geometrical Isomerism (Cis-Trans)

In Square Planar Complexes [MA₂B₂]

Example: [Pt(NH₃)₂Cl₂]

     Cis Isomer                  Trans Isomer

     Cl—Pt—NH₃                   Cl—Pt—Cl
        |                            |
     Cl  NH₃                      NH₃ NH₃

     (cisplatin)                 (transplatin)
     Anticancer                  Inactive

Characteristics:

In Square Planar [MA₂BC]

Example: [Pt(NH₃)(NH₂OH)(Cl)(Py)]

Three different geometrical isomers are possible!

In Square Planar [MABCD]

All four ligands different → Three isomers possible

Geometrical Isomerism in Octahedral Complexes

Type 1: [MA₄B₂]

Example: [Co(NH₃)₄Cl₂]⁺

        Cis Isomer               Trans Isomer

        NH₃                          NH₃
         |                            |
    Cl—Co—NH₃                    Cl—Co—Cl
         |                            |
        Cl                           NH₃

    (B's adjacent)               (B's opposite)

Properties:

Type 2: [MA₃B₃]

Example: [Co(NH₃)₃Cl₃]

Two isomers:

• Facial (fac): Three same ligands on one triangular face
• Meridional (mer): Three same ligands in a plane through metal

        Fac Isomer                 Mer Isomer

        All 3 Cl                   3 Cl form a plane
        on one face                through the center

Type 3: [MA₂B₂C₂]

Maximum 5 geometrical isomers possible!

Optical Isomerism

Isomers that are non-superimposable mirror images (enantiomers).

Required conditions:

1. No plane of symmetry
2. No center of symmetry
3. Molecule is chiral

Recognition Test

A complex is optically active if:

• It cannot be superimposed on its mirror image
• It lacks a plane of symmetry

Octahedral Complexes with Bidentate Ligands

Example: [Co(en)₃]³⁺

           Δ-isomer                    Λ-isomer
       (right-handed)              (left-handed)

    Non-superimposable mirror images!
    Both optically active

These are called Δ (delta) and Λ (lambda) forms.

Properties:

• Rotate plane-polarized light in opposite directions
• Same chemical formula, same physical properties (except optical rotation)
• Equal stability

Another Example: cis-[Co(en)₂Cl₂]⁺

• cis form: Optically active (two enantiomers exist)
• trans form: NOT optically active (has plane of symmetry)

Quick Summary Table

Memory Tricks

“IHLLCO” for Structural Isomers

• Ionization
• Hydrate
• Linkage
• Ligand
• Coordination
• (skip to) Optical and geometrical (stereoisomers)

Geometrical Isomer Quick Check

Square Planar:

• MA₂B₂ → 2 isomers (cis, trans)
• MA₂BC → 3 isomers
• MABCD → 3 isomers

Octahedral:

• MA₄B₂ → 2 isomers (cis, trans)
• MA₃B₃ → 2 isomers (fac, mer)
• MA₂B₂C₂ → up to 5 isomers

Optical Activity Memory

“T-PLANE” Rule:

• Trans isomers have a PLANE of symmetry
• Therefore: NEVER optically active!

Common Mistakes

Practice Problems

Level 1: Identification

Q1. Identify the type of isomerism: a) [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br b) [Pt(NH₃)₂Cl₂] (cis and trans) c) [Co(NH₃)₅NO₂]Cl₂ and [Co(NH₃)₅ONO]Cl₂ d) [Cr(H₂O)₆]Cl₃ and [Cr(H₂O)₅Cl]Cl₂·H₂O

Q2. Which of these can show geometrical isomerism? a) [Pt(NH₃)₂Cl₂] b) [Ni(NH₃)₆]²⁺ c) [Co(NH₃)₄Cl₂]⁺ d) [NiCl₄]²⁻

Q3. Which can show optical isomerism? a) trans-[Co(en)₂Cl₂]⁺ b) cis-[Co(en)₂Cl₂]⁺ c) [Co(NH₃)₆]³⁺ d) [Co(en)₃]³⁺

Level 2: Application

Q4. How many geometrical isomers are possible for [Co(NH₃)₃(NO₂)₃]?

Q5. A complex [Co(NH₃)₅X]Y₂ exists in two forms:

• Form A gives white precipitate with AgNO₃
• Form B gives white precipitate with BaCl₂

Identify X and Y, and name the type of isomerism.

Q6. Draw all possible geometrical isomers of [Pt(NH₃)(Br)(Cl)(py)] (square planar).

Q7. Which octahedral complex will show BOTH geometrical and optical isomerism? a) [Co(NH₃)₆]³⁺ b) [Co(en)₃]³⁺ c) [Co(en)₂Cl₂]⁺ d) [Co(NH₃)₄Cl₂]⁺

Level 3: JEE Advanced

Q8. How many stereoisomers (including optical) are possible for [Co(en)₂Cl₂]⁺?

Q9. A coordination compound with molecular formula CrCl₃·6H₂O can exist in three isomeric forms:

• Isomer A: Loses no water on heating, gives 3 moles AgCl
• Isomer B: Loses 1 mole water easily, gives 2 moles AgCl
• Isomer C: Loses 2 moles water easily, gives 1 mole AgCl

Write structural formulas for all three isomers.

Q10. Explain why: a) [Pt(NH₃)₂Cl₂] shows geometrical isomerism but [Pt(NH₃)₃Cl]⁺ doesn’t b) cis-[Pt(NH₃)₂Cl₂] is polar but trans-[Pt(NH₃)₂Cl₂] is non-polar c) [Co(en)₃]³⁺ is optically active but [Co(NH₃)₆]³⁺ is not

Q11. How many total isomers (structural + stereoisomers) are possible for [Co(en)₂(NO₂)Cl]⁺?

Q12. A student claims that [MA₂B₂C₂] (octahedral) can have 15 different isomers (including all types). Is this claim correct? Explain.

Solutions to Selected Problems

Q1. a) Ionization b) Geometrical c) Linkage d) Hydrate

Q2. a) Yes (square planar) c) Yes (octahedral) b) No (all same ligands) d) No (tetrahedral, all Cl)

Q3. b) Yes (cis, no plane of symmetry with bidentate ligands) d) Yes (chiral with three bidentate ligands) a) No (trans has plane of symmetry) c) No (all same ligands, highly symmetrical)

Q4. 2 isomers (fac and mer)

Q5. Form A: X = Cl, Y = SO₄ → [Co(NH₃)₅Cl]SO₄ Form B: X = SO₄, Y = Cl → [Co(NH₃)₅SO₄]Cl₂ Type: Ionization isomerism

Q7. c) [Co(en)₂Cl₂]⁺

• Has cis/trans (geometrical)
• cis form is optically active (2 enantiomers)

Q8. 3 stereoisomers total:

• 1 trans (not optically active)
• 2 cis (Δ and Λ, optically active pair)

Q9.

• Isomer A: [Cr(H₂O)₆]Cl₃
• Isomer B: [Cr(H₂O)₅Cl]Cl₂·H₂O
• Isomer C: [Cr(H₂O)₄Cl₂]Cl·2H₂O

Real-World Applications

1. Drug Design

• Cisplatin vs Transplatin: Geometric isomers with vastly different activities
• Carboplatin: Modified cisplatin with better properties
• Understanding isomerism = designing better drugs!

2. Catalysis

• Different isomers have different catalytic activities
• Wilkinson’s catalyst: Specific geometry required for hydrogenation

3. Biological Systems

• Hemoglobin: Oxygen binding changes geometry
• Vitamin B₁₂: Complex has specific optical configuration
• Chlorophyll: Magnesium complex with specific geometry

4. Materials Science

• Magnetic properties depend on geometry (cis vs trans)
• Color depends on isomeric form
• Crystal packing different for different isomers

Visual Summary

Isomerism Decision Tree

graph TD
    A[Same molecular formula?] -->|Yes| B[Same bonding pattern?]
    A -->|No| Z[Not isomers]

    B -->|No| C[Structural Isomerism]
    B -->|Yes| D[Stereoisomerism]

    C --> C1[Different ions?
→ Ionization]
    C --> C2[H₂O in/out?
→ Hydrate]
    C --> C3[Different binding atom?
→ Linkage]
    C --> C4[Ligands exchanged?
→ Coordination]

    D --> D1[Different spatial arrangement?
→ Geometrical cis/trans]
    D --> D2[Mirror images?
→ Optical Δ/Λ]

Related Topics

Within Coordination Compounds

• Werner’s Theory — Why coordination compounds exist
• Nomenclature — Naming cis, trans, and optical isomers
• Bonding Theories — Why certain geometries form
• Crystal Field Theory — Effect of geometry on properties

Cross-Chapter Connections

• Chemical Bonding — VSEPR and molecular geometry
• d-Block Elements — Metals that form coordination compounds
• Organic Chemistry - Isomerism — Similar concepts in organic compounds

Interactive Demo: Visualize Coordination Isomers

See different isomeric forms of coordination compounds in 3D (cis-trans, optical isomers).