When solutions containing two (or) more simple stable compounds in molecular proportions are allowed to evaporate, crystals of new substances are obtained.
These substances are termed molecular (or) addition compounds. Some common examples are :
K2SO4 + Al2(SO4)3 + 24H2O —> K2SO4.Al2(SO4)3.24H2O
CuSO4 + 4NH3 —> CuSO4.4NH3
The molecular (or) addition compounds are of two types :
(i) Double salts : The addition compounds which are stable in solid state only but are broken down into individual constituents when dissolved in water are called double salts. Their solutions have the same properties as the mixture of individual compounds.
When Mohr’s salt , [FeSO4.(NH4)2SO4.6H2O] is dissolved in water it exhibits the properties of FeSO4 and (NH4)2SO4
i.e. they produce Fe2+, NH4+ and SO42– ions in solution. Thus each ion has its identity in double salt.
(ii) Co-ordination compounds : The addition compounds in which some of the constituent ions (or) molecules lose their identity and when dissolved in water they do not break up completely into individual ions are called co-ordination compounds.
The properties of their solutions are different than those of their constituents. When CuSO4.4NH3 are dissolved in water there is hardly any evidence for the presence of Cu2+ ions (or) NH3 molecules.
A new ion [Cu(NH3)4]2+ is furnished in which NH3 molecules are directly linked with the metal ion
CuSO4.4NH3 ↔ [Cu(NH3)4]2+ + SO42-
Thus, a co-ordination compound may be defined as a molecular compound that results from the combination of two (or) more simple stable molecular compounds and retains its identity in the solid as well as in dissolved state.
The properties of such compounds are totally different than individual constituents. A co-ordination compound contains very often but not always a complex ion.
A complex ion may be defined as an electrically charged radical which consists a central metal atom (or) ion surrounded by a group of ions (or) neutral molecules (or) both.