Question

write werners theory ?


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Answer 1

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.

Answer 2

Hey !!

Alfred Werner was the first to formulate his ideas about the structures of coordination compounds. He prepared and characterised a large number of coordination compounds and studied their physical and chemical behaviour by simple experimental techniques.

 Werner proposed the concept of primary and secondary valence for a metal ion. In a series of compounds of cobalt(III) chloride with ammonia, it was found that some of the chloride ion could be precipitated as silver chloride on adding excess silver nitrate solution but some chlorides remained unprecipitated.

One mole CoCl₃.6NH₃ (yellow) gave three mole AgCl, one mole CoCl₃.5NH₃ (purple) gave two mole AgCl, one mole CoCl₃.4NH₃ (green) gave one mole AgCl. These observations together with the results of conductivity measurements in solution can be explained if six groups in all, either chloride ions or ammonia molecules or both, remain bonded to the cobalt ion during the reaction.

              Werner proposed the term secondary valence for the number of groups bound directly to the metal ion.

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