Question

Define ligand. Write four postulates of warner’s theory?

Answer 1

Hint: A ligand is an ion or molecule that binds to a central atom to form a coordination complex. Werner put forth this theory to describe the structure and formation of complex compounds or coordination compounds. According to Werner's theory, a coordination compound has two different types of valency, primary and secondary. The primary valence or the ionisable valency is satisfied by the negative charged ions in the solution.

Complete answer:
Ligands are ions or neutral molecules that bond to a central metal atom or ion. Ligands act as Lewis bases (electron pair donors), and the central atom acts as a Lewis acid (electron pair acceptor). Ligands have at least one donor atom with an electron pair used to form covalent bonds with the central atom.
Postulates of Werner’s theory:
-Central metal ions in a complex show two types of valances: primary valence and secondary valence.
-The primary valence is ionisable and satisfied by negative ions
-Secondary valence is non ionisable.It is equal to the coordination number of the central metal ion or atom. It is fixed to a metal. Secondary valencies are satisfied by negative ions or neural molecules.
-The primary valence is non-directional. The secondary valence is directional. Ions or molecules attached to satisfy secondary valences have characteristic spatial arrangements. Secondary valence decides the geometry of the complex compound.

Note:
The common limitations of Werner's theory are: It could not explain the inability of all elements to form coordination compounds. Werner's theory could not explain the directional properties of bonds in various coordination compounds.