Werner's Coordination Theory

Werner's coordination theory and it's experimental verification:

In 1893, Werner was the first to propose correct structures for coordination compounds containing complex ions , in which a central transition metal atom is surrounded by neutral or anionic ligands .

CoCl₃•6NH₃ This type of representation does not implies any proper geometry around the central metal ion

The basic postulates of Werner's theory may be summerised as follows:

1. In co-ordination compounds, central metal atoms exhibit primary valency and secondary valency.

The primary valency is ionizable. Secondary valency is not ionizable.The primary valency corresponds to oxidation state.The secondary valency corresponds to coordination number. (the central metal ion and ligands are not ionizable)


2. Every metal atom has a fixed number of secondary valencies (coordination number(s)).


3. The metal atom tends to satisfy boths its primary valency as well as its secondary valency. Primary valency is satisfied by negative ions (metal ion has a positive charge) whereas secondary valency (coordination number) is satisfied either by negative ions or by neutral molecules. (In certain case a negative ion may satisfy both types of valencies).


4. The coordination number or secondary valencies are always directed towards the fixed positions in space and this leads to definite geometry of the coordination compound.

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, 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.





​​​​​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.









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References


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