Coordination Compounds

Introduction and Werner’s Theory of Coordination Compounds

Who is the father of coordination chemistry? Yes! The man in question is none other than Werner. He is famous for his Werner’s theory of coordination compounds. Are you aware of what this theory is? In this chapter, we will study all about the theory and look at its postulates and examples.

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What is Werner’s Theory?

In 1823, Werner put forth this theory to describe the structure and formation of complex compounds or coordination compounds. It is because of this theory that he got the Nobel prize and is known as the father of coordination chemistry. Are you ready to learn the important postulates of this theory?

Postulates of Werner’s Theory

The important postulates of Werner’s theory are:

  • The central metal or the metal atoms in coordination compounds show two types of valency. They are the primary and secondary valency.
  • The primary valency relates to the oxidation state and the secondary valency relates to the coordinate number.
  • The number of secondary valences is fixed for every metal atom. It means that the coordination number is fixed.
  • The metal atom works towards satisfying both its primary and secondary valencies. A negative ion satisfies the primary valency. On the other hand, a negative ion or neutral molecules satisfy secondary valencies.
  • The secondary valencies point towards a fixed position in space. This is the reason behind the definite geometry of the coordinate compound. For example, let us consider the case of a metal ion having six secondary valencies. These arrange octahedrally around the central metal ion. If the metal ion has four secondary valencies, these arrange in either tetrahedral or square planar arrangement around the central metal ion. Therefore, we see that the secondary valency determines the stereochemistry of the complex ion. On the other hand, the primary valency is non-directional.

Learn the Nomenclature of Coordination Compounds here.

Examples Based on Postulates of Werner’s Theory

Werner’s theory is responsible for the formation of structures of various cobalt amines. We will look at its explanation now. Cobalt has a primary valency (oxidation state) of three and exhibits secondary valency (coordination number) of 6. We represent the secondary valencies by thick lines and the primary valency by broken lines.

1) CoCl3.6NH3 Complex: In this compound, the coordination number of cobalt is 6 and NH3 molecules satisfy all the 6 secondary valencies. Chloride ions satisfy the 3 primary valencies. These are non-directional in character. These chloride ions instantaneously precipitate on the addition of silver nitrate. The total number of ions, in this case, is 4, three chloride ions and one complex ion.

Werner's Theory

2) CoCl3.5NH3 complex: In this compound, cobalt has the coordination number of 6. However, we see that the number of NH3molecule decreases to 5. The chloride ion occupies the remaining one position. This chloride ion exhibits the dual behaviour as it has primary as well as secondary valency.

Werner's Theory

3) CoCl3.4NH3 complex: In this compound, two chloride ions exhibit the dual behaviour of satisfying both Primary and Secondary Valencies. This compound gives a precipitate with silver nitrate corresponding to only one Cl ion and the total number of ions, in this case, is 2. Hence, we can formulate it as [CoCl2(NH3)4]Cl.

Werner’s Theory and Isomerism

Werner turned his attention towards the geometrical arrangements of the coordinated groups around the central cation. He was successful in explaining the cause behind optical and geometrical isomerism of these compounds. Some examples are as follows:

1) [CoCl2(NH3)4]Cl complex: According to Werner, there are three structures possible for this complex. These are planar, trigonal prism, octahedral. The number of possible isomers is 3 for planar, 3 for trigonal prism and 2 for octahedral structure.

However, as we could isolate only two isomers of the compound, he concluded that geometrical arrangement of the coordinated group around the central atom in this compound was octahedral. In the case of several other complexes in which the coordination number of the central atom was six, Werner was of the opinion that in all these cases the six coordinated complex have octahedral geometry.

He also read the geometry of the complexes where the coordination number of the central metal atom is 4. He gave two possible structures for such compounds: Square Planar and Tetrahedral. Let us look at an example of the same.

2) [PtCl2(NH3)2] complex: In this complex, the coordination number of the metal is 4. According to Werner, this complex exists in two isomeric forms, cis and trans. This shows that all the four ligands lie in the same plane. Therefore, the structure should be a square planar or tetrahedral.

Learn more about Isomerism and its types.

Solved Example for You

Q: What are the limitations of Werner’s Theory?

Ans: Like all the major theories, Werner’s Theory was not free from limitations. The common limitations of the theory are:

  • It could not explain the inability of all elements to form coordination compounds.
  • The Werners theory could not explain the directional properties of bonds in various coordination compounds.
  • It does not explain the colour, the magnetic and optical properties shown by coordination compounds.

Q. What is IUPAC name of [Fe(NH3)4O2C2O4]Cl?

Learn the Nomenclature of Coordination Compound to find the answer.

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