Werner's Theory of Coordination Compounds: Introduction, Postulates and Examples

Alfred Werner, a Swiss chemist, was the first person to make attempts to formulate the structure of the coordination compounds in the year 1892. He conducted different sorts of experiments such as precipitation experiments, conductive experiments, etc., to come up with a new theory, which is today popular as Werner's Theory. He is therefore also known as the father of coordination chemistry. 

According to this theory, any metal in the coordinate compound possesses two types of valencies. In this chapter, we are going to discuss Werner's Theory in detail and its various postulates, Valence Bond Theory and its postulates and limitations etc. This chapter is a part of Coordination Compounds and carries a total weightage of 6 to 7 marks and 8 periods.

Keyterms: Valence Bond Theory, Coortdination Compounds, Crystal Field theory, d-orbitals, Quantum mechanics, Covalent Bonds

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Werner's Theory of Coordination Compounds-Definition

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Werner put a theory that successfully explained the formation and structure of complex compounds or coordinate compounds which came to be known as Werner's Theory.  Along with it, he also introduced the concept of primary valence and secondary valence of metal ions. We are going to have a detailed discussion about each topic in the next sections:

Figure: Werner’s representation of coordination compounds

Postulates of Werner's Theory

The important postulates as observed by Alfred Werner throughout his experiments are as follows:

• The complex/ coordination compounds contain a central metal atom.
• The metal atoms in a coordination compound generally show two types of valency: primary valency and secondary valency.
• The primary valencies denote the oxidation state. They are ionizable and are satisfied by the negative ions.
• Secondary valencies denote the coordination number. They are non-ionizable and are fixed for every metal atom. The secondary valency is generally satisfied by the neutral molecules or negative ions.
• The metal atoms should satisfy both primary and secondary valencies.
• The secondary valency of the atom basically shows the geometry/ polyhedra of the particular coordination compound.

Drawbacks of Werner's Theory

Even though Werner's theory was successful in describing the reasons behind the formation of coordination compounds, yet it left a few drawbacks as mentioned below:

•  Werner's theory failed to give a valid explanation as to why only a few elements are able to form coordination compounds.
• It failed to describe why the bonds of a coordination compound show directional properties.
• It does not have any explanation for the magnetic properties, optical properties, and color found in these coordination compounds.

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Valence Bond Theory

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The Valence Bond Theory was introduced with the objective to explain various chemical bondings through quantum mechanics. According to the VB Theory, the electrons present in a molecule inhabit the atomic orbitals instead of molecular orbitals. It states that the metal atom/ ions in a covalent bond under the influence of ligands share electrons through the overlapping of their valence atomic orbitals (n-1)d, ns, np, or ns, np, nd. This sharing of electrons takes place in order to achieve equivalent orbitals of a certain defined geometry. 

Figure: Valence bond theory

Postulates of Valence Bond Theory

The postulates of the valence bond theory are as follows:

• When two different atoms with valence orbitals overlap with each other, it results in the formation of covalent bonds with the ligands. 
• The area between both bonding atoms experiences an increase in the electron density due to the overlapping of the atoms. 
• The remaining unpaired electrons in the valence shell form bonding with multiple atoms. However, the paired atoms in the valence shell do not participate in any further bonding with atoms. 
• Covalent bonds show directional properties. A hybridized orbital shows better directional characteristics compared to an unhybridized one. 

Limitations of the Valence Bond Theory

Despite all the success, the valence bond theory falls short in many cases. The limitations of the VB theory are as follows:

• It fails to describe the spectral properties of the complex compound. 
• It does not provide any distinction between the strong and weak ligands. 
• The VB theory fails to explain the tetravalent property exhibited by carbon. 
• It does not explain the color shown by the complex/ coordinate compounds. 
• The theory does not provide a quantitative explanation of the thermodynamic or kinetic stability of the compounds. 

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Crystal Field Theory

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According to the Crystal Field Theory, ligands are nothing but point charges, and the metal-ligand bond is an ionic bond that possesses an electrostatic nature. It states that the five 'd-orbitals' of transition metals degenerate, i.e., they have the same energy when not bonded to any ligand.

Figure: Crystal field theory

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Factors behind the splitting of d-orbitals

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The splitting of the d-orbitals in the complex compounds is observed basically due to the following factors:

• The nature of the ligand
• The nature of the metal atom
• Geometry of the compound (octahedral/ tetrahedral)
• Oxidation state of the specific metal ion.

Werner's theory laid a great way to understand the concept behind the formation and structures of coordination compounds. The observations during Werner's experiments led to the introduction of the primary and secondary valency and how they described the characteristics of a particular complex compound. Later, the Valence bond theory and Crystal Field theory provided a greater picture of the atomic orbitals and the nature of the metal-ligand ionic bonds. 

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