In 1893, Alfred Werner developed a theory for complex compound. Werner was able to explain nature of bonding in complexes.
The important postulates of Werner coordination theory are as following.
- In Coordination compounds, the metal atom posses two types of valencies primary valancy and secondary valency.
Primary valancy corresponds to oxidation number of metal ion is reffed as ionizable valancy. Secondary valancy is ion and is referred valency Is ionisable valency. It also more or less fixed depending upon central metal atom. A number of ligand surrounds the central metal atom. A number of ligand surrounds the central metal atom i.e. coordination number corresponds to the secondary valancy of metal.
- Every metal atom or ion has more or less fixed number of coordination numbers.
- Ligand donates electron pair to the central metal atom. Ligands are commonly negative ion such as Cl–or neutral molecule containing lone pairs of electron e.g NH3.
- Secondary valences are directional in nature so that complex ions have a fixed shaped depending upon the coordination number. For eg. if coordination number is six, the geometry of complex is octrahedral like [Co(NH3)6]3+.