Werner's Theory of Coordination Compounds

Introduction

In 1893, Alfred Werner revolutionized chemistry by proposing a theory that explained the mysterious “complex compounds” that had puzzled chemists for decades. His work was so groundbreaking that he became the first inorganic chemist to win the Nobel Prize in Chemistry (1913).

The Mystery Before Werner

Before 1893, chemists were puzzled by compounds like:

• CoCl₃·6NH₃ (yellow crystals)
• CoCl₃·5NH₃ (purple crystals)
• CoCl₃·4NH₃ (green/purple crystals)

Questions that needed answers:

1. Why do these compounds exist?
2. Why do they have different colors despite similar formulas?
3. How are NH₃ molecules attached to CoCl₃?
4. Why do they show different conductivities?

Werner’s Brilliant Insight

Werner realized that metal atoms have TWO types of valencies:

Primary Valency (Ionizable)

• Satisfied by negative ions (anions)
• Corresponds to the oxidation state of the metal
• Shows ionic character
• Non-directional in nature
• Represented by dotted lines in diagrams

Secondary Valency (Non-ionizable)

• Satisfied by ligands (neutral molecules or anions)
• Equals the coordination number
• Shows covalent character
• Directional in nature - leads to specific geometry
• Represented by solid lines in diagrams

Werner’s Postulates

Postulate 1: Dual Valencies

Metals exhibit two types of linkages in coordination compounds - primary and secondary valencies.

Postulate 2: Primary Valency

• Ionizable valency
• Satisfied by negative ions
• Depends on the oxidation state of metal
• Non-directional

Postulate 3: Secondary Valency

• Non-ionizable valency
• Satisfied by neutral molecules or negative ions
• Equals the coordination number
• Directional - gives definite geometry
• Fixed for a particular metal

Postulate 4: Spatial Arrangement

The ligands attached through secondary valencies are arranged in specific geometrical patterns around the central metal:

Understanding Through Examples

Example 1: CoCl₃·6NH₃ (Yellow)

Werner’s Interpretation:

[Co(NH₃)₆]Cl₃

Analysis:

• Primary valency of Co³⁺ = 3 (satisfied by 3 Cl⁻ ions)
  • These 3 Cl⁻ are outside the coordination sphere
  • They ionize in solution
• Secondary valency of Co³⁺ = 6 (satisfied by 6 NH₃)
  • These 6 NH₃ are inside the coordination sphere
  • They form coordinate bonds
• Geometry: Octahedral
• Conductivity: High (produces 4 ions: 1 cation + 3 anions)

Interactive Demo: Visualize Werner’s Complex Structures

See how ligands arrange around the central metal atom in 3D coordination complexes.

graph TD
    A[Co³⁺] --> B1[NH₃]
    A --> B2[NH₃]
    A --> B3[NH₃]
    A --> B4[NH₃]
    A --> B5[NH₃]
    A --> B6[NH₃]
    C1[Cl⁻] -.-> A
    C2[Cl⁻] -.-> A
    C3[Cl⁻] -.-> A

    style A fill:#3498db
    style B1 fill:#2ecc71
    style B2 fill:#2ecc71
    style B3 fill:#2ecc71
    style B4 fill:#2ecc71
    style B5 fill:#2ecc71
    style B6 fill:#2ecc71
    style C1 fill:#e74c3c
    style C2 fill:#e74c3c
    style C3 fill:#e74c3c

Example 2: CoCl₃·5NH₃ (Purple)

Werner’s Interpretation:

[Co(NH₃)₅Cl]Cl₂

Analysis:

• Primary valency = 3 (but only 2 Cl⁻ outside)
  • 1 Cl⁻ is inside the coordination sphere
• Secondary valency = 6 (5 NH₃ + 1 Cl⁻)
• Geometry: Octahedral
• Conductivity: Moderate (produces 3 ions: 1 cation + 2 anions)

Example 3: CoCl₃·4NH₃ (Green/Purple)

Werner’s Interpretation:

[Co(NH₃)₄Cl₂]Cl

Analysis:

• Primary valency = 3 (only 1 Cl⁻ outside)
• Secondary valency = 6 (4 NH₃ + 2 Cl⁻)
• Geometry: Octahedral
• Conductivity: Low (produces 2 ions: 1 cation + 1 anion)
• Shows isomerism (cis and trans forms)

Experimental Evidence Supporting Werner

1. Conductivity Measurements

Werner measured the molar conductivity of these compounds:

2. Precipitation with AgNO₃

When treated with AgNO₃ solution, only the Cl⁻ ions outside the coordination sphere precipitate as AgCl:

3. Isomerism Studies

Werner predicted and synthesized geometric isomers of [Co(NH₃)₄Cl₂]Cl:

• cis isomer (green)
• trans isomer (purple)

This proved the octahedral geometry and directional nature of secondary valency!

Modern Understanding

Today, we understand Werner’s theory in modern terms:

Memory Tricks

The “PSIN” Rule

Primary = State of oxidation = Ionizable = Non-directional

Secondary = Spatial arrangement = Stable bonds = Specific geometry

Counting Ions Trick

For [M(ligands)ₓ]Yₙ:

• Cation ions = 1
• Anion ions = n
• Total ions = n + 1

Example: [Co(NH₃)₆]Cl₃ → 1 + 3 = 4 ions

Common Mistakes

Practice Problems

Level 1: Basic Understanding

Q1. Identify the primary and secondary valencies in: a) [Cr(NH₃)₆]Cl₃ b) K₄[Fe(CN)₆] c) [Pt(NH₃)₂Cl₂]

Q2. How many ions are produced when [Co(NH₃)₅Br]SO₄ dissolves in water?

Q3. How many moles of AgCl will precipitate when excess AgNO₃ is added to 1 mole of [Co(NH₃)₄Cl₂]Cl?

Level 2: Application

Q4. A coordination compound of chromium has the formula CrCl₃·5H₂O. Upon treatment with excess AgNO₃, 2 moles of AgCl precipitate per mole of compound. Write: a) The correct structural formula b) The IUPAC name

Q5. Two compounds have the formula PtCl₄·2NH₃. One is yellow and non-conducting, while the other is pale yellow and conducts electricity. Explain using Werner’s theory and write their structural formulas.

Q6. A complex of cobalt(III) with formula CoCl₃·4NH₃ exists in two isomeric forms (violet and green). One form produces 1 mole of AgCl with AgNO₃ while the other produces 2 moles. Explain with structures.

Level 3: JEE Advanced

Q7. A coordination compound with molecular formula CrCl₃·6H₂O gives:

• No precipitate with AgNO₃
• Produces 2 ions in solution (molar conductivity similar to CaCl₂)

Write the structural formula and explain the bonding.

Q8. How would you experimentally distinguish between:

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

Q9. A platinum(IV) compound has the molecular formula PtCl₄·2NH₃·2KCl. Werner proposed three possible structures. Using conductivity and precipitation experiments, how would you determine the actual structure?

Solutions to Selected Problems

Q1. a) Primary = 3, Secondary = 6 b) Primary = 2, Secondary = 6 c) Primary = 2, Secondary = 4

Q2. 2 ions ([Co(NH₃)₅Br]²⁺ and SO₄²⁻)

Q3. 1 mole of AgCl

Q4. a) [Cr(H₂O)₅Cl]Cl₂·H₂O b) Pentaaquachloridochromium(III) chloride monohydrate

Historical Impact

Werner’s theory:

• Explained the structure of thousands of “complex” compounds
• Predicted and confirmed geometric isomerism
• Laid foundation for understanding biological molecules
• Led to the development of coordination chemistry as a field
• Earned Werner the Nobel Prize in Chemistry (1913)

Related Topics

Within Coordination Compounds

• Nomenclature — How to name coordination compounds systematically
• Isomerism — Geometric and optical isomers Werner predicted
• Bonding Theories — Modern explanation using VBT

Cross-Chapter Connections

• d-Block Elements — Properties of transition metals
• Chemical Bonding — Coordinate covalent bonds
• Ionic Equilibrium — Conductivity and ion formation

Coordination Compounds | Nomenclature →