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Werner in 1898, propounded his theory of coordination compounds. In coordination compounds metals show two types of linkages (valences)-primary and secondary. The primary valences are normally ionisable and are satisfied by negative ions. The secondary valences are non-ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal. The ions or group bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to the different coordination numbers. Both double salts as well as complexes are formed by the combination of two or more stable compounds in stoichiometric ratio. However, they differ in the fact that double salts such as carnallite, Mohr's salt, potash alum etc. dissociate into simple ions completely when dissolved in water. But complex ions do not dissociate completely into simple ions.
When \(1 mol CrCl _3 \cdot 6 H _2 O\) is treated with excess of \(AgNO _3, 3 mol\) of AgCl are obtained. The formula of the complex is:

1. A \([CrCl_{3}(H_{2}O)_{3}] \cdot 3H_{2}O\)
2. B \([CrCl_{2}(H_{2}O)_{4}]Cl \cdot 2H_{2}O\)
3. C \([CrCl(H_{2}O)_{5}]Cl_{2} \cdot H_{2}O\)
4. D \([Cr(H_{2}O)_{6}]Cl_{3}\)