merits and demerits of werners theory

Merits of Werner’r theory
  1. Werner’s theory explains the structure and function of co-ordination compounds.
  2. Unlike simple compounds, all the ions in co-ordination compounds do not ionize in solution.
This concept is explained by Werner, in terms of primary and secondary valency or linkages in these compounds. The primary valencies are ionisable while the secondary valencies are not.  The atoms, ions or molecules forming the secondary valency lie closest to the central metal atom and the complex of metal and these groups act as one single unit.  The number of bonds these entities form with the metal atom is called the co-ordination number of the compound.  These secondary linkages are written with the metal atom within a square bracket.
For example, [Co(NH3)6]Cl3. The six ammonia groups in square brackets are linked by secondary valencies to the central metal atom, Co.  As there are 6 such molecules, the co-ordination number is 6 and these do not ionize in aqueous solution. The three chlorine atoms outside the square bracket are the primary valencies/linkages and these ionize when the compound is dissolved in water.
In [CoCl(NH3)5]Cl2,  the five ammonia molecules and one chlorine atom are the secondary valencies while the two chlorine atoms written outside the square brackets are the primary linkages. As there are a total of six secondary linkages, co-ordination number is 6.
  1. Werner’s theory correctly predicted that the central atom and secondary linages define the molecular geometry of the compound. The theory correctly postulated that octahedral, tetrahedral and square planar geometric shapes are most common in these co-ordination compounds.
Demerits of Werner’s theory
  1. The theory presumes that the primary valencies are always negative ions. This is true for most of the co-ordination compounds but some exceptions exist. For example, potassium ferrocyanide, K4[Fe(CN)6], the primary valency is satisfied by potassium cations.
  2. The theory fails to correlate the bonding and spatial arrangement of ions, atoms or molecules in a co-ordination compound with respect of the electronic configuration of the central metal atom.
  3. The theory does not give an explanation regarding the colour and magnetic properties of co-ordination compounds.

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