Werner's theory

Werner’s Theory of Coordination Compounds

Werner's theory, proposed by Alfred Werner in 1893, revolutionized the understanding of coordination compounds. It explains the structure, bonding, and behavior of metal complexes, laying the foundation of modern coordination chemistry. Werner was later awarded the Nobel Prize in Chemistry in 1913 for this work.

Key Postulates of Werner’s Theory

• Primary Valency: It corresponds to the oxidation state of the central metal ion and is satisfied by negatively charged ions (e.g., Cl⁻, NO₃⁻).
• Secondary Valency: It represents the number of ligands attached to the central metal atom and is satisfied by neutral molecules or anions (such as H₂O or NH₃) that are directly coordinated to the metal.
• Fixed Coordination Number: Each metal has a characteristic and fixed number of secondary valencies.
• Directional Nature: Secondary valencies are directional in nature and responsible for the geometry of the complex (e.g., octahedral, tetrahedral).

Example: [Co(NH₃)₆]Cl₃

In this complex:

• Cobalt has a primary valency of 3 (satisfied by 3 Cl⁻ ions).
• It also has a coordination number of 6 (satisfied by 6 NH₃ molecules).
• All 6 NH₃ molecules are directly bonded to Co³⁺, forming the coordination sphere.

Types of Linkages

Werner’s theory distinguishes between ions within and outside the coordination sphere. The species enclosed within the coordination sphere (brackets) remain intact in solution and do not ionize, while the ions outside the brackets behave like typical electrolytes and can dissociate.

Significance of Werner’s Theory

• Explains the difference between ionizable and non-ionizable parts of a complex compound.
• Explains the number of ligands a central metal ion can accommodate and the resulting spatial arrangement.
• Provided a foundation for understanding and predicting different types of isomerism in coordination compounds.

Limitations

Although Werner’s theory provided a strong foundation, it does not explain the nature of bonding or why certain ligands form more stable complexes. These aspects are addressed by later theories like Valence Bond Theory and Crystal Field Theory.