Werner’s Theory of Coordinate Compounds

What is Werner’s Theory?

A very famous scientist Werner’s in the year 1823 put forward his theory of coordination compounds which describes the formation and structure of complex compounds which is later termed as Werner’s Theory of Coordinate Compounds.

Due to this theory he is awarded by Nobel prize and he is also called the ‘Father of Coordination Chemistry’.

Postulates of Werner’s Theory

The important postulates of Werner’s theory are as follows:

• In coordination compounds, the central metal or metal atoms exhibit two types of valency, For Example: The primary valency and the secondary valency. The primary valency corresponds to oxidation state and the secondary valency corresponds to coordinate number.

• Every metal atom has a fixed number of secondary valencies, For Example: It has fixed coordinate number.

• The metal atom tends to satisfy its both primary and secondary valancies. Primary valency is satisfied by negative ion whereas secondary valancies are satisfied by negative ion or by neutral molecules.

• The secondary valancies are always directed towards fixed position in space and this cause definite geometry of the coordinate compound. For Example: If a metal ion has six secondary valencies, these are arranged octahedrally around the central metal ion. If the metal ion has four secondary valencies, these are arranged in either tetrahedral or square planar arrangement around the central metal ion. The secondary valency thus determines the stereochemistry of the complex ion. On the other hand, the primary valency is non-directional.

Examples Based on Postulates of Werner’s Theory

Structures of various cobalt ammines is based on Werner’s theory is given below:

Cobalt has a primary valency (oxidation state) of three and exhibit secondary valency (coordination number) of 6. The secondary valencies are represented by thick lines and the primary valency is represented by broken lines.

• CoCl₃.6NH₃ Complex: In this compound, the coordination number of cobalt is 6 and all the 6 secondary valencies are satisfied by NH₃ molecules (the black solid lines). The 3 primary valencies are satisfied by chloride ions (the dotted line in fig). These are non-directional in character. These chloride ions are instantaneously precipitated on the addition of silver nitrate. The total number of ions in this case is 4, three chloride ions and one complex ion. The central ion and the neutral molecules or ions satisfying secondary valencies are written in a square bracket while writing the formula of the compound. Hence the complex may be written as [Co(NH₃)₆]Cl₃ and as shown as in fig. 

• CoCl₃.5NH₃ complex: In this compound the coordination number of cobalt is also 6 but the number of NH₃ molecule is decreased to 5 from 6 and one remaining position is now occupied by chloride ion. This chloride ion exhibits the dual behaviour as it has primary as well as secondary valency. Secondary valency is shown by full line and the primary valency is shown by dotted line in the figure. 

This structure satisfies the 3 primary and 6 secondary valencies of cobalt. Hence the complex formed may be formulated by writing five ammonia molecules and one chloride ion inside the square brackets and the two chloride ions outside the brackets [CoCl(NH₃)₅]Cl_2.

• _{​​​​​}CoCl₃.4NH₃ complex: In this compound, two chloride ions exhibit dual behaviour of satisfying both Primary and Secondary Valencies. This compound will give precipitate with silver nitrate corresponding to only one Cl^- ion and the total number of ions in this case is 2. Hence it can be formulated as [CoCl₂(NH₃)₄]Cl.

• CoCl3.3NH3 complex: In this compound, three chloride ions satisfy primary as well as secondary valency. No Cl^- will be precipitated on the addition of silver nitrate at room temperature. Therefore, the complex compound behave as neutral non conducting molecule. It may be formulated as [CoCl₃(NH3)₃].

Werner’s Theory and Isomerism

Werner turned his attention towards the geometrical arrangements of the coordinated groups around the central cation and explained successfully the cause of optical and geometrical isomerism of these compounds. Some examples are given below:

• [CoCl2(NH3)4]Cl: Werner said that theoretically there are three structure possible for this complex. These are planar, trigonal prism, octahedral. The number of possible isomer is 3 for planar, 3 for trigonal prism and 2 for octahedral structure.

Fig. various possible isomers for the planar, trigonal prism and octahedral structures of the complex ion [CoCl3(NH3)4]

since only two isomers of the compound could be isolated, werner 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 able to conclude that in all these cases the six coordinated complex have octahedral geometry.

He studied the geometry of the complexes in which the coordination number of the central metal atom is 4. He proposed that there are two possible structure. Square Planar and Tetrahedral.

• [PtCl2(NH3)2] complex: In this complex the coordination number of the metal is 4, werner found that it existed 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.

Fig. Cis and trans isomers of [PtCl2(NH3)2] complex
 

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