Write postulates of Werner theory of co-ordination complexes. Write the name of a hexadentate ligand.

Answer:

Postulates of Werner's theory:

1. The central metal atom in a complex shows two types of valencies: primary valency and secondary valency.
2. Primary valency corresponds to the oxidation state of the metal. It is ionisable and is satisfied by negative ions (anions).
3. Secondary valency corresponds to the coordination number of the metal. It is non-ionisable and is satisfied by neutral molecules or negative ions (ligands).
4. Every metal has a fixed number of secondary valencies (fixed coordination number).
5. Secondary valencies are directed in space towards fixed positions around the metal, which gives the complex a definite geometry, e.g. octahedral for coordination number 6, tetrahedral or square planar for 4.

e.g. in [Co(NH₃)₆]Cl₃, the primary valency of Co is 3 (satisfied by 3Cl⁻) and secondary valency is 6 (satisfied by 6NH₃).

Hexadentate ligand: ethylenediaminetetraacetate ion (EDTA⁴⁻).