Werner's Theory and It's Limitations

Werner’s Theory

In 1893, Werner produced a theory to explain the structures, formation and nature of bonding in the coordination compounds. This theory is known as Werner's theory of coordination compounds.
Some important postulates of this theory are given below-
1. The central metal atoms in coordination compounds show two types of valency. first one is the primary valency (Principal or Ionizable) and the second one is the secondary valency (Auxiliary or non-Ionizable).
2. The primary valency relates to the oxidation state and the secondary valency relates to the coordinate number.
3. The number of secondary valences is fixed for every metal atom. that means the coordination number is fixed.
4. Central metal atom satisfy both its primary and secondary valencies. Primary valency is satisfied by negative ion whereas secondary valancies are satisfied by negative ion or by neutral molecules.
5. The secondary valancies are always directed towards fixed position in space and this cause definite geometry of the coordinate compound. For examples: If a metal ion has six secondary valencies, these are arranged octahedrally around the central metal ion. If the metal ion has four secondary valencies, these are arranged in either tetrahedral or square planar arrangement around the central metal ion. The secondary valency thus determines the stereochemistry of the complex ion. While the primary valency is non-directional.
6. The secondary valencies are generally represented by solid lines while the primary valencies are represented by dashed lines and the ions which satisfy both primary and secondary valencies will be drawn with both solid and dashed lines.

Limitations of Werners Theory

1. It could not explain the inability of all elements to form coordination compounds.
2. The Werners theory could not explain the directional properties of bonds in various coordination compounds.
3. It does not explain the colour, the magnetic and optical properties shown by coordination compounds.

Q. According to the postulates of Werner's theory for coordination compounds

a. primary valency is ionizable
b. secondary valency is ionizable
c. primary and secondary valencies are non-ionizable
d. only primary valency is non-ionizable

Q. Which of the following postulates of Werner's theory is incorrect

a. Secondary valence is equal to the coordination number and it depends upon the nature of ligand attached to metal.
b. The ions/ groups bound by the secondary linkages to the metal have charecteristic spatial arrangements
c. Primary valencies are satisfied by negative ions
d. None of these

Q. When AgNO₃ is added to a solution of Co(NH₃)₃Cl₃, the precipitate of AgCl shows two ionisable chloride ions. This means

a. Two chlorine atoms satisfy primary valency and one secondary valency
b. One chlorine atom satisfies primary as well as secondary valency
c. Three chlorine atoms satisfy primary valency
d. Three chlorine atoms satisfy secondary valency

Q. According to Werner's theory the geometry of the complex is determined by

a. Only from the primary valence in space
b. Number and position of the primary valences in space
c. Number and position of the secondary valences in spaced
d. Only from the position of secondary valence In space