Werner’s Theory of Coordination Compounds

  

            The fundamental postulates of Werner's theory which was actually a result of dream and made Werner        a noble prize winner in chemistry may be summarized as below.

       1. Metals possess two types of valencies, namely primary (principal) or ionizable valency and secondary (auxiliary) 

     or non–ionizable valency.

  a-  Primary valencies are those which a metal normally exhibit in the formation of its simple salts. Thus, in the formation

     of PtCl₄, CuSO₄ and AgCl the primary valencies of Pt, Cu and Ag are 4, 2 and 1 respectively. Primary valencies are

        satisfied by negative ions.

  b- Secondary Valencies are those which a metal cation exercises towards a neutral molecule or negative group in the 

            formation of its complex ions. Thus, secondary valencies may be satisfied by negative ions, neutral molecules

           having lone electron pair (e.g., H₂O, NH₃, etc.) or even sometimes by some positive groups. In every case, the

           coordination number of the metal must be fulfilled.

       

COORDINATION COMPOUNDS OF COCL3

₃ WITH NH3₃

      In modern terminology the primary valency corresponds to oxidation number and the secondary to coordination
     number. Primary valencies are shown by dotted lines while secondary by thick lines.

2. Every metal has a fixed number of secondary valencies, e.g., cobalt3+ and platinum4+ were recognized to have
 six secondary valencies and copper2+ four. The total number of secondary valencies required by a metal is more
  commonly known as coordination number (C.N).

3. The secondary valencies are always directed towards fixed positions in space about the central metal ions.
   Thus, the number and arrangement of ligands in space determines the stereochemistry of a complex. Thus,
    in case of six secondary valencies the arrangement of secondary valencies may be as directed to the apices of
   a regular octahedral while in case of four secondary valencies arrangement might be either a planar or a tetrahedral
  manner and thus, this postulate predicted the possibilities of a variety of type of isomerism in coordination compounds.
 Remember that primary valencies are non–directional.