Comprehension

Werner’s coordination theory in 1893 was the first attempt to explain the bonding in coordination complexes. It must be remembered that this theory was put forward before the electron had been discovered by J.J. Thomson in 1897, and before the electronic theory of valency. Werner did not have any of the modern instrumental techniques and all his studies were made using simple experimental techniques. Werner was able to explain the nature of bonding in complexes and he concluded that in complexes, the metal shows two different sorts of valency: primary and secondary. Primary valences are normally ionisable whereas secondary valences are non-ionisable. 

Question: 1

One mole of CrCl3·4H2O precipitates one mole of AgCl when treated with excess of AgNO3 solution. Write (i) the structural formula of the complex, and (ii) the secondary valency of Cr.

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The secondary valency refers to the number of coordinate bonds formed between the metal ion and the ligands.
Updated On: Jul 11, 2025
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Solution and Explanation

(i) The structural formula of the complex is: \[ \chemfig{CrCl_3 \cdot 4H_2O} \] (ii) The secondary valency of Cr in this complex is 6, as there are six coordination bonds formed between the chromium ion and chloride ions and water molecules.
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Question: 2

What is the difference between a complex and a double salt?

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A complex has metal-ligand coordination, while a double salt forms via simple ionic bonds.
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Solution and Explanation

A complex consists of a metal ion bonded to ligands via coordinate bonds, whereas a double salt is a combination of two simple salts that crystallize together without involving coordination bonds. In complexes, the metal ion has a well-defined oxidation state and coordination number, while double salts lack these bonding interactions.
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Question: 3

Arrange the following complexes in the increasing order of their conductivity of their solution: [Cr(NH3)3Cl]3, [Cr(NH3)6Cl]3, [Cr(NH3)5Cl]Cl2.

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The more ions that dissociate in solution, the greater the conductivity.
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Solution and Explanation

The order of conductivity is as follows: \[ [Cr(NH_3)_6Cl]_3>[Cr(NH_3)_5Cl]Cl_2>[Cr(NH_3)_3Cl]_3 \] This is due to the number of ions produced in solution; the more ions produced, the higher the conductivity.
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Question: 4

Write two differences between primary and secondary valences in coordination compounds.

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Primary valence is concerned with the ionic nature, while secondary valence defines coordination geometry.
Updated On: Jul 11, 2025
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Solution and Explanation

1. Primary valence refers to the oxidation state of the metal ion, while secondary valence refers to the number of coordinate bonds formed by the metal ion with ligands.
2. Primary valence is ionizable and determines the charge on the complex, whereas secondary valence is non-ionizable and determines the coordination number of the metal ion.
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