A coordination compound is a complex compound which contains a metal bonded to ligands. A ligand can be an atom, an ion or a molecule that donates electrons to the metal. They were known since 18th century but no theory was developed till that time which could explain for the properties of these compounds. In the year 1898, Alfred Werner proposed the Werner’s theory explaining the structure of coordination compounds. He carried out a series of experiments on cobalt (III) chloride and ammonia. Upon addition of silver nitrate solution he observed that some of the chloride ions precipitated as silver chloride. Further, he also observed that the amount of silver chloride formed was related to the number of ammonia molecules bound to the cobalt (III) chloride. For example, when silver nitrate was added to CoCl3·6NH3, all the three chloride ions were converted to silver chloride. However, when silver nitrate was added to CoCl3·5NH3, only two mole of silver chloride was formed. When CoCl3·4NH3 was treated with silver nitrate, one mole of silver chloride was formed. Based on his observations, Werner postulated the following theory:
- The central metal atom in a coordination compound exhibits two types of valences (linkages) – primary and secondary.
- Primary valences are generally ionisable and are satisfied by negative ions.
- The secondary valences are non-ionisable. These are generally satisfied by negative ions or neutral molecules. The secondary valence is fixed for a metal and is equal to the coordination number.
- The ions bounded by the secondary linkages to the metal exhibit characteristic spatial arrangements corresponding to different coordination numbers.
Such spatial arrangements are now known as coordination polyhedra. The species within the square brackets are called coordination entities or complexes and the ions outside the square brackets are known as counter ions.
Limitations of Werner’s Theory:
- It failed to explain why all elements don’t form coordination compounds.
- It failed to explain the directional properties of bonds in coordination compounds.
- It does not explain the colour, and the magnetic and optical properties shown by coordination compounds.
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