Molecular or addition compounds are formed when stoichiometric amounts of two or more stable compounds join together. For example:
KCl + MgCl2 + 6H2O → KCl.MgCl.6H2O (Carnallite)
K2SO4 + Al2 (SO4)3 + 24H2O → K2SO4.Al2(SO4)3.24H2O (potassium alum)
CuSO4 + 4NH3 + H2O → CuSO4.4NH3H2O
(tetrammine copper (II) sulphate monohydrate)
Fe(CN2) + 4KCN → Fe(CN)2.4KCN (potassium ferrocyanide)
These fall into two categories:
(1) Those which lose their identity in solution.
(2) Those which retain their identity in solution.
An aqueous solution of carnallite shows the properties of K+, Mg2+ and Cl- ions. Potassium alum solutions similarly show the properties of K+, Al3+ and
SO42– ions. They are called double salts and exist only in the crystalline state.
The other two examples of addition compounds behave in a very different way from the double salts. When dissolved they do not form Cu2+, or Fe2+ and CN- ions, but instead give more complicated structures-the cuproammonium ion [Cu(H2O)2(NH3)4]2+ and the ferrocyanide ion [Fe(CN)6]4-. These are complex ions and exist are a single entity. Complex ions are indicated by square brackets. Molecular compounds of this type are called coordination compounds.
Co-ordination compounds are the compounds in which the central metal atom is linked to ions or neutral molecules by co-ordinate bonds, e.g.
[Cr(H2O)5Cl]2+. If the species thus formed as given above carries positive charge, it is called a complex ion.
Home