Werner was able to explain the bonding in complex.
Primary valency (Pv) : This is non- directional and ionizable. In fact it is the positive charge on the metal ion.
Secondary valency (Sv) : This is directional and non- ionizable. It is equal to the number of ligand atoms co-ordinated to the metal (co-ordination number).
Example : [Co(NH3)6]Cl3 | or | Co(NH3)6]3+3Cl– |
Pv → 3Cl–[3] | Sv | → 6NH3(6) |
[Co(NH3)5Cl]Cl2
Pv → 2Cl–(2) |
or
Sv |
[Co(NH3)5Cl2+2Cl–
→5NH3 + 1Cl– (6) |
[Co(NH3)4Cl2Cl | or | [Co(NH3)4Cl2]+Cl |
Pv → Cl–(1) | Sv | →4NH3 + 2Cl–(6) |
Nature of the complex can be understood by treating the above complexes with excess of AgNO3.
CoCl3. 6NH3 → 3AgCl, [Co(NH3)6Cl3 (three chloride ion)
CoCl3. 5NH3 → 2AgCl, [Co(NH3)5Cl2 (two chloride ion)
CoCl3. 4NH3 → 1AgCl, [Co(NH3)4Cl2 (one chloride ion)
CoCl3. 3NH3 → no AgCl, [Co(NH3)3Cl3 (no chloride ion)
The nature of bonding between central metal atom and ligands in the coordination sphere has been explained by the three well-known theories. These are :
Coordination Compounds : Valence Bond theory of Coordination Compounds