Werner’s Theory Of Coordination Compounds

1.0Introduction

Werner’s Coordination Theory was proposed in 1898 by Swiss chemist Alfred Werner. Through extensive studies on coordination compounds' physical, chemical, and isomeric properties, Werner formulated key postulates that explain their bonding and structure.

This article provides an overview of Werner’s Coordination Theory, its key postulates, and a detailed discussion of primary and secondary valencies.

2.0Postulates  of Werner’s Theory 

Werner proposed the following fundamental concepts regarding coordination compounds:

1. In a coordination compound, the central metal atom exhibits two types of valencies:

• Primary valency (ionizable)
• Secondary valency (non-ionizable)

2. Primary valencies correspond to the oxidation state of the metal and are satisfied by negative ions (e.g., Cl⁻, SO₄²⁻).
3. Secondary valencies correspond to the coordination number of the metal and are satisfied by either neutral molecules (e.g., NH₃, H₂O) or negative ions (e.g., CN⁻, Cl⁻).
4. The number of secondary valencies (coordination number) is fixed for each metal, and the spatial arrangement of ligands around the metal is determined.
5. The ligands attached via secondary valencies form a definite geometric shape known as a polyhedron.

• Common geometries in coordination compounds include tetrahedral, octahedral, and square planar arrangements.
• The ligands arranged around the central metal ion occupy fixed positions in space, giving the complex a definite geometry.
• For example, in CoCl₃·5NH₃, one Cl⁻ shifts from primary valency to secondary valency.
• As a result, only two Cl⁻ ions remain ionizable, while 5 NH₃ molecules and 1 Cl⁻ ligand form coordinate bonds with Co³⁺.

3.0Types of Valencies

Metals exhibit two types of valencies: Primary valency and Secondary valency.

Primary Valencies (Oxidation State)

• Primary valencies are ionizable and correspond to the oxidation state of the metal.
• They are observed in simple salts such as CoCl₃, CuSO₄, and AgCl.
• For example, in CoCl₃, CuSO₄, and AgCl, the primary valencies of Co, Cu, and Ag are +3, +2, and +1, respectively.

Secondary Valencies (Coordination Number)

• Secondary valencies are non-ionizable and determine the coordination number of the metal.
• These are satisfied by ligands (neutral molecules or ions) that form coordinate bonds with the metal.
• Examples of coordination complexes:
• • [Co(NH₃)₆]³⁺ → Coordination number = 6
  • [Cu(NH₃)₄]²⁺ → Coordination number = 4
  • [Ag(NH₃)₂]⁺ → Coordination number = 2

4.0Werner's Representation Of Complexes

Key Notations:

• Dotted lines (…..) → Represent Primary Valency (Ionizable).determines ionizability.
• Continuous lines (——) → Represent Secondary Valency (Non-ionizable).determines the geometry and coordination number.
• As the number of ionizable Cl⁻ ions decreases, the ionization and electrical conductivity of the compound decrease.

1. $[Co(N{H}_3{)}_6]C{l}_3$

• Primary valency: 3 (satisfied by 3 Cl⁻ counter ions, ionizable).
• Secondary valency: 6 (satisfied by 6 NH₃ molecules, non-ionizable).
• Structure: Octahedral (since six NH₃ ligands surround the Co³⁺ ion)

2. $[Co(N{H}_3{)}_{5}Cl]C{l}_2$

• Primary valency: 2 (satisfied by 2 Cl⁻ counter ions, ionizable).
• Secondary valency: 6 (satisfied by 5 NH₃ molecules and 1 Cl⁻ ligand, non-ionizable).
• Structure: Octahedral

• Here, one Cl⁻ acts as a secondary valency (inside the brackets), while the other two Cl⁻ ions act as primary valencies (outside the brackets and ionizable).

3. $[Co(N{H}_{₃}{)}_{₄}C{l}_{₂}]Cl$

• Primary valency: 1 (satisfied by 1 Cl⁻ counter ion, ionizable).
• Secondary valency: 6 (satisfied by 4 NH₃ molecules and 2 Cl⁻ ligands, non-ionizable).
• Structure: Octahedral

• One Cl⁻ is ionisable (primary valency, outside the brackets).
• Two Cl⁻ ligands are inside the brackets (secondary valencies, non-ionizable).

5.0Experimental Evidence Supporting Werner's Theory

Several experimental observations, including precipitation reactions and electrical conductance studies, support Werner’s coordination theory.

(a) Precipitation of Primary Valencies Using a Suitable Reagent

• When a coordination compound is treated with AgNO₃ solution, the ionizable chloride ions (Cl⁻) react with silver ions to form an AgCl precipitate.
• Example:
• • [Fe(NH₃)₆]Cl₃ produces 3 moles of AgCl upon reaction with AgNO₃, confirming that the three Cl⁻ ions are ionizable and exist outside the coordination sphere.
  • The complex dissociates as: $\text{[Fe(NH₃)₆]Cl₃}\to {\text{[Fe(NH₃)₆]}}^{3+}+3C{l}^{-}$

(b) Electrical Conductance of Complexes

• The electrical conductivity of a coordination compound in an aqueous solution depends on the number of free ions present.
• The greater the number of ions, the higher the electrical conductivity.
• Example:
• • The electrical conductance of an aqueous solution of [Fe(NH₃)₆]Cl₃ is higher than that of [Fe(NH₃)₅Cl]Cl₂ since the former releases more ions in solution.
  • Dissociation of [Fe(NH₃)₅Cl]Cl₂ in water: $\text{[Fe(NH₃)₅Cl]Cl₂}\to {\text{[Fe(NH₃)₅Cl]}}^{2+}+2C{l}^{-}$
  • Only three ions are produced here, leading to lower conductivity than [Fe(NH₃)₆]Cl₃, which dissociates into four ions.