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# Explain Werner’s theory of coordinate compounds with suitable examples?

Last updated date: 20th Jun 2024
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Hint: Coordinate compounds are those compounds that remain in solid-state even when dissolved in a liquid. In such compounds, central metal atoms or ions are connected with other ions or molecules (called ligands) with coordinate bonds.

We know coordinate compounds are the compounds that remain in solid-state even when dissolved. Werner was the first one to explain the coordinate theory. Postulates of his theory rewritten below:
1. In coordination compounds, the central metal atom possesses two types of valency: primary valency and secondary valency. Primary valency represents oxidation state (charge) and secondary valency represents coordinate number (molecules to which central metal atom is linked). For example in ${K_4}\left[ {Fe{{\left( {CN} \right)}_6}} \right]$ central atom is iron. It is linked with $6$ $CN$ molecules which represent its secondary valency i.e. secondary valency of iron is$6$. As each $CN$molecule possess $- 1$ charge, on whole coordinate sphere charge is $- 4$ (each potassium atom has charge of $- 1$ and as a whole compound is neutral so there will be $- 4$charge on anionic part) so charge on iron will be $+ 2$, which represents primary valency of iron.
2. Every metal atom has a fixed number of secondary valencies. This means the coordination number of the central atom is fixed as it will make bonds according to the number of electrons in the outermost shell.
3. Each central metal atom tends to satisfy its both valency primary as well as secondary. Primary valency is defined as a charge which will be satisfied by negative ions only as in the above example $CN$ has a negative charge. But secondary valency can be satisfied by both negative ions and neutral molecules. As neutral molecules have lone pairs to donate.
4. As secondary valency is always directed towards the centre the coordinate complex has definite geometry. For example: in the complex written above, the secondary valency of the atom is $6$ which will be arranged in octahedral geometry around the central atom. Similarly, if a complex will have $4$ secondary valency ligands will be arranged in either tetrahedral or square planar geometry.
So, this is the explanation of Werner’s theory.

Note:
Werner’s theory has some limitations as well which are as follows:
He was unable to explain why only certain elements make coordinate compounds.
Why bonds are directional in nature.
Why coordination compounds possess magnetic and optical properties.