Introduction to Co-Ordination Compounds

 

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 SO₄^2– 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.

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