Werner’s Theory of Coordination Compounds

Full Notes

Alfred Werner, a Swiss chemist, won the Nobel Prize in Chemistry in 1913 for his work on the structure of coordination compounds. In 1898, he put forward a theory that explained the properties and structures of these compounds.

Werner observed that when certain metal compounds were dissolved in water, some negatively charged ions separated and became free in the solution. He referred to these as primary valences.

However, he also noticed that some groups or molecules present in the compound’s formula did not dissociate in solution. Instead, they remained firmly attached to the central metal atom. He termed these as secondary valences.

For Example
When CoCl₃•6NH₃ is dissolved in water, Werner found that chloride ions (Cl⁻) were released into the solution, but ammonia molecules (NH₃) were not. This suggested that the NH₃ molecules remained bonded to the cobalt ion. In this case, the Cl⁻ ions represent the primary valences, while the NH₃ molecules represent the secondary valences.

Werner’s Main Postulates

Werner summarised his findings into four key points.

• In coordination compounds, metals show two types of linkages (valences) – primary and secondary.
• The primary valences are normally ionisable and are satisfied by negative ions.
• The secondary valences are non-ionisable. These are satisfied by neutral molecules or negative ions. The secondary valency is equal to the coordination number and is fixed for a metal.
• The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers.

Explanation of Terms

Some of these are covered in more detail in section 5.2

Primary Valency:
Equivalent to the oxidation state of the metal. These are ionisable and typically satisfied by anions such as Cl⁻, SO₄²⁻, NO₃⁻, etc.

Secondary Valency:
Corresponds to the coordination number (CN) of the metal. These are non-ionisable and are satisfied by ligands, which may be neutral (e.g., NH₃, H₂O) or negatively charged (e.g., Cl⁻, CN⁻). Ligands are linked to the central metal via coordinate bonds and determine the geometry of the complex.

Coordination Number:
The total number of ligand donor atoms bonded to the central atom via coordinate bonds. This number is fixed for a given metal in a given oxidation state.

Geometry:
The spatial arrangement of ligands around the metal ion. Common geometries include:

• Octahedral (CN = 6)
• Tetrahedral or square planar (CN = 4)

Illustrative Examples of Werner Complexes

Example CoCl₃·6NH₃

Structural formula: [Co(NH₃)₆]Cl₃
Coordination number: 6 (satisfied by 6 NH₃ molecules)
Primary valency (oxidation state of Co): +3 (satisfied by 3 Cl⁻ ions outside the bracket)
Dissociation in aqueous solution:
[Co(NH₃)₆]Cl₃ → [Co(NH₃)₆]³⁺ + 3Cl⁻
3 Cl⁻ ions precipitate with AgNO₃
Conductivity: 4 ions in solution

Example CoCl₃·5NH₃

Structural formula: [Co(NH₃)₅Cl]Cl₂
Coordination number: 6 (5 NH₃ + 1 Cl⁻ inside bracket)
Primary valency: +3 (2 Cl⁻ outside the bracket)
Dissociation:
[Co(NH₃)₅Cl]Cl₂ → [Co(NH₃)₅Cl]²⁺ + 2Cl⁻
2 Cl⁻ ions precipitate with AgNO₃
Conductivity: 3 ions in solution

Example CoCl₃·4NH₃

Structural formula: [Co(NH₃)₄Cl₂]Cl
Coordination number: 6 (4 NH₃ + 2 Cl⁻ inside bracket)
Primary valency: +3 (1 Cl⁻ outside the bracket)
Dissociation:
[Co(NH₃)₄Cl₂]Cl → [Co(NH₃)₄Cl₂]⁺ + Cl⁻
1 Cl⁻ ion precipitates with AgNO₃
Conductivity: 2 ions in solution

Distinction Between Double Salts and Complex Compounds

Werner also distinguished between double salts and coordination complexes based on their behaviour in aqueous solution.

• Double Salts
  • Formed by crystallising two or more salts together in a definite stoichiometric ratio.
  • Dissociate completely into all constituent ions in solution.
  • All ions are detectable by standard chemical tests.
  • Exist only in solid state.
  • ExampleK₂SO₄·Al₂(SO₄)₃·24H₂O → 2K⁺ + 2Al³⁺ + 3SO₄²⁻ + 24H₂O
• Complex Compounds
  • Contain a central metal atom or ion surrounded by ligands in a coordination sphere.
  • The complex ion does not dissociate in solution; only the ions outside the sphere do.
  • Exhibit distinct geometry and properties.
  • Exist both in solid and solution state.
  • ExampleK₄[Fe(CN)₆] → 4K⁺ + [Fe(CN)₆]⁴⁻ (The [Fe(CN)₆]⁴⁻ ion remains intact.)

Comparison Table

Summary

• Primary valency is ionisable and equals the oxidation state.
• Secondary valency is non-ionisable and equals the coordination number.
• Ligand arrangement defines fixed geometry around the metal.
• Werner’s theory explains conductivity, ionisation and isomerism.
• Double salts dissociate fully but complexes keep the coordination entity intact.