Alfred Werner, a well-known scientist, proposed his theory of coordination compounds in 1823, which describes the formation and structure of complex compounds and is now known as Werner’s Theory of Coordinate Compounds. He was given the Nobel Prize for this hypothesis. Werner is known as the ‘Father of Coordination Chemistry’.

Postulates of Werner’s Theory

The  postulates of Werner’s theory are as follows:

1. The central metal or metal atoms in coordination compounds have two types of valency, namely, primary valency and secondary valency. 
2. The oxidation state is represented by the primary valency, whereas the coordinate number is represented by the secondary valency.
3. Every metal atom has a predetermined number of secondary valencies, such as a fixed coordinate number. 
4. The primary and secondary valencies of a metal atom tend to be satisfied. Negative ions satisfy primary valencies, but neutral molecules or negative ions satisfy secondary valencies.
5. The secondary valencies are always directed towards a fixed position in space, resulting in the definite geometry of the coordinate compound.  Consider the following example: The secondary valencies of a metal ion are grouped octahedrally around the central metal ion if it possesses six. If the metal ion possesses four secondary valencies, they are grouped around the central metal ion in a tetrahedral or square planar structure. The stereochemistry of the complex ion is thus determined by the secondary valency. The primary valency, on the other hand, is non-directional.

Examples for postulates of Werner’s theory

The following are the structures of several cobalt amines based on Werner’s theory:

• Cobalt has a primary valency of three (oxidation state) and a secondary valency of six (coordination number). The primary valency is represented by broken lines, whereas the secondary valencies are represented by thick lines.
• CoCl3.6NH3 Complex: The coordination number of cobalt in this compound is 6, and NH3 molecules satisfy all six secondary valencies (the black solid lines). Chloride ions satisfy the three primary valencies (the dotted line in fig). These have a non-directional.  When silver nitrate is added, the chloride ions precipitate very instantly. In this scenario, there are four ions total: three chloride ions and one complex ion. While writing the compound’s formula, the central ion and neutral molecules or ions satisfying secondary valencies are enclosed in square brackets.

Werner’s theory and isomerism

Werner turned his attention to the geometrical configurations of the coordinated groups around the central cation, which he effectively explained as the cause of these compounds’ optical and geometrical isomerism. 

The following are some examples:

• [CoCl2(NH3)4]Cl: According to Werner, there are three potential structures for this complex. Planar, trigonal prism, and octahedral are the three types. There are three possible isomers for a planar structure, three for a trigonal prism, and two for an octahedral structure.
• The transition metals can be found in a variety of complex compounds in which the metal atoms are bonded to a large number of anions or neutral molecules. Such compounds are commonly referred to as coordination compounds in modern terminology. In modern inorganic chemistry, the chemistry of coordination compounds is an important and complex area. This feature, however, is not exclusive to transition metals and can be found in small amounts in a variety of other metals. (Example: Chlorophyll is a coordination compound of magnesium).

Applications of Werner’s theory

1. Werner’s theory predicts the exact structure of each complex.
2. It explains why a specific metal atom and, more specifically, a specific ligand form different  complexes. It also explains the different properties of each complex, with C.N. 4 and 6, it predicts the formation of various compounds.
3. The final postulate of Werner’s theory explained isomerism and anticipated the existence of isomerism of forms that had never been observed before. Werner discovered that the divalent platinum complex [Pt(NH3)2Cl2] exists in both cis and trans isomeric forms.

Limitations of Werner’s theory

Werner’s theory of coordination compounds has some limitations, which are as follows: 

1. It was unable to explain why all elements were unable to form coordination complexes.
2. The bonding nature between the central metal atom and the ligands was not explained by Werner’s coordination theory.
3. When secondary valency was equal to 4, Werner’s coordination theory failed to explain the geometry of complexes.
4. It explains nothing about the colour, magnetic, or optical properties of coordination compounds.

Conclusion

Since the 18th century, coordination compounds have been known. However, no suitable hypothesis could be found to explain these compounds’ observable features. Alfred Werner proposed the idea of auxiliary (secondary) valency in 1893 in order to provide a right explanation for the properties of coordination compounds. Werner proposed a theory that explained the structures, formation, and nature of coordination molecules’ bonding. Werner’s theory of coordination compounds is the name for this concept. 

Werner was awarded the Nobel Prize in Chemistry in 1913, making him the first inorganic chemist to do so. He investigated a number of complex compounds produced by the interaction of cobalt chloride with ammonia.  However, Werner couldn’t characterise the colour of the coordinate complex. He couldn’t explain the magnetic and optical properties of coordination compounds.