Werner's Theory

What is Werner’s Theory of Coordination Compounds? 

In 1893 Werner produced a theory to explain the structures, formation and nature of bonding in the coordination compounds. This theory is known as Werner’s theory of coordination compounds.

Werner was the first inorganic chemist to be awarded the Nobel Prize for chemistry in 1913. He studied many complex compounds obtained from the reaction between cobalt chloride and ammonia.

Table of Contents

• Postulate
• Explanation
• Evidence in favour Werner’s theory
• Limitation of Werner’s theory
• Heteroleptic Complex
• Frequently Asked Questions – FAQs

The Postulate of Werner’s Theory

The central metals of coordination compounds exhibit two types of valencies

1. Primary Valency
2. Secondary valency

 1. Primary Valency

• Primary valencies are those which a metal exhibits in the formation of simple salts. e.g. CoCl₃ , NaCl, CuSO₄ etc.
• In modern terminology it represents the oxidation number of metal.

e.g. the primary valencies of Co in CoCl₃ is 3,  and oxidation state +3 

Similarly, for NaCl, oxidation state of Na is +1, for CuSO₄, oxidation state of Cu is +2

• The primary valencies are ionizable.
• These are written outside the coordination sphere.
• These are non-directional and do not give any geometry to complex compound

Example: [Co(NH₃)₆] Cl₃, number of primary valencies 3, oxidation state +3

2. Secondary Valency

• The secondary valency of metals is either by negative ions or neutral molecules or both.
• In modern terminology it represents the coordination number of the metal.
• Secondary valencies are written inside the coordination sphere.
• These are directional in nature and give definite geometry to the complex.
• These are non-ionisable.

Example: [Co (NH₃)₆]Cl₃ coordination number is 6.

Explanation of Structure and Properties on the basis of Werner’s Theory:

• Werner explained the Structure and properties of following four complexes of Co (III) chloride with ammonia.

• Werner added the above four cobalt(III) complexes in table with an excess silver nitrate solution. Which resulted in different amounts of silver chloride precipitate.

• He concluded that in CoCl₃.6NH₃, 3Cl^– ions react with 3 silver ion form 3 silver chloride precipitates which act as primary valency and 6NH₃ molecules act as secondary valency. Thus in modern term the compound is written as [Co(NH₃)₆] Cl₃.
• Similarly, for CoCl₃. 5NH₃,  2Cl^– ions act as primary valency, the remaining 1Cl^– and 5NH₃ ions act as secondary valency. So the compound is  [Co(NH₃)₃Cl] Cl₂
• For CoCl3.4NH3, 1Cl^– is primary valency, 2Cl^– and 4NH₃ is secondary valency. So the compound is   [Co(NH₃)₅ Cl₂] Cl
• For CoCl3.3NH3, all 3Cl^– and 3NH₃ ion act as secondary valency So the compound is   [Co(NH₃)₅ Cl₃]
• He represents the primary valencies by dotted lines(…….) and secondary valencies by solid line (—)

Evidence in favour of Werner’s theory

1. Cryoscopic Measurement:

The cryoscopic measurement (i.e., measurement of depression in freezing point) gives the number of ions formed by the dissociation of an ionic compound. The depression in freezing point is a colligative property and depends upon the number of particles in the solution. The greater the number of particles, the more will be the freezing point.

2. Electrical Conductance Measurement:

The conductance of solution depends upon the numbers of charge particles present in that solution.

[Co(NH₃)₆]Cl₃ → [Co(NH₃)₆]⁺  +  3Cl–

[Co(NH₃)₅Cl]Cl₂ → [Co(NH₃)₅Cl]⁺  + 2Cl^–

[Co(NH₃)₄Cl₂]Cl → [Co(NH₃)₄Cl₂]⁺  +  Cl^–

So that the molar conductance of the compound should be the following order which also satisfies the observed conductance value.

[Co(NH₃)₆]Cl₃  >   [Co(NH₃)₅Cl]Cl₂  >   [Co(NH₃)₄Cl₂]Cl  >   [Co(NH₃)₃Cl₃]

3. Precipitation Reaction:

On the addition of silver nitrate solution, with chloride complex. The chloride ions which present outside the coordination sphere undergo precipitation reaction. As the number of chloride ions present outside the sphere increases, the number of formation of precipitates increases and vice versa. 

Limitation of Werner’s theory

• Though Werner explained some properties of the coordination compound,  he failed to explain the colour of the coordinate compound.
• He could not explain the magnetic and optical properties of coordination compounds.
• He could not answer the question, why does the coordination sphere have a definite geometry.

Heteroleptic Complex

Heteroleptic complexes: Heteroleptic complexes are those in which a metal ion is surrounded by more than one type of ligand. Heteroleptic complexes are those in which the metal atom or ion is connected to more than one type of ligand. These are complexes in which a metal is bonded solely by ore-type ligands, such as [Co(NH₃)₆]³⁺ Complexes heteroleptic: These are complexes in which a metal is bound by multiple types of ligands, such as [Co(NH₃)₄]Cl₂]⁺.