What is Werner's theory
Werner's Theory (1823) describes the structure and formation of complex compounds. According to him, transition metal cations have two types of valencies: primary and secondary. The primary valence is equal to the metal cation's oxidation number, while the secondary valence is equal to the coordination number. Sone important difference between primary and secondary valencies are given in table below-
| S. No. | Primary Valencies | Secondary Valencies |
|---|---|---|
| 1 | They correspond to the oxidation numbers of the central metal atom or ion | They correspond to the co-ordinate numbers of the central metal atom or ion |
| 2 | They are ionisable | They are non-ionisable |
| 3 | They lead to ionic bonds | They lead to covalent bonds |
| 4 | They are variable | They are fixed. e.g. Fe(II), Fe(III),Cr(III) etc. have secondary valency six and Pt(II), Cu(II), Cd(II) etc. have secondary valency four |
| 5 | They refer to the transfer of electrons | They refer to electron pair sharing |
| 6 | Group attached to primary valencies are shown outside the square bracket, [ ] | Group attached to secondary valencies are well within the square bracket, [ ] |
| 7 | They are non directional in nature | They are directional in nature. Therefore, they point out the geometry of complexes |
| 8 | They must be always satisfied by anions | They may be satisfied by anoin, neutral molecules or even positively charged groups. e.g. NO+ |
| 9 | They don’t attribute to geometrical isomerism | They do attribute to geometrical isomerism |
| 10 | Weak | Strong |
| 11 | Unstable | Stable |
It is interesting to see how these postulates can be and were used to explain the behavior of a typical complex hexamine cobalt (III) chloride, [Co(NH3)6]Cl3 in the light of werner’s theory. Here the central cobalt atom is in +3 state, hence it has three primary valency. Since three Cl− ions are attached to primary valencies which are ionisable, hence these ions can be precipitated by adding AgNO3 solution as AgCl. These Cl− ions are shown outside the square bracket. There will not be any ionization of [Co(NH3)6]+3 ion as all the six NH3 molecules are attached by secondary valencies to the metal ion. Conductance measurements in aqueous solution of this complex show the presence of only four ions. Thus according to werner's theory, there should be three Cl− ions and one complex ion as a result of ionization of this complex:
[Co(NH3)6]Cl3 ⇌ [Co(NH3)6]+3 + 3Cl−
Further secondary valency of the metal is six, hence it is an octahedral complex ion, as shown in figure
