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Werner's Theory of Coordination compounds- Theory, Experimental Evidence, Postulates, Polyhedra, Practice Problems and FAQs
Assume you're in 1893, and there are two cyanide compounds, 
and 
kept before you. One is a very dangerous poison, while the other is not.. Can you guess?
If not, KCN is the poisonous material. But the other compound
, in spite of having 6 times the -CN group is not poisonous.Why?
Alfred Werner solved this problem. He said that, salt, , is completely dissociated when dissolved in water, yielding free cyanide ions, while the other salt is a complex ions, such as 
of , do not dissociate to give free - 
ions to act as a poison..
Table of Contents
- Werner's theory of Coordination compounds
- Experimental shreds of evidence
- Postulates of Werner’s Theory
- Coordination Polyhedra
- Limitations of Werner’s theory
- Practice Problems
- Frequently Asked Questions
Werner's theory of Coordination compounds
Werner developed a theory in 1893 to explain the structures, formation, and nature of bonding in coordination compounds. First of all, let's discuss the experiment performed by Werner.
Experimental shreds of evidence
When salt 
is mixed with 
, there is a formation of different coloured compounds.
(The last two compounds have the same empirical formula, 
but different properties. These compounds are known as isomers.)
While adding an excess of 
solution in a series of compounds of Co(III) chloride with , the following results were found:
| Precipitation Reaction | No moles of precipitates | Conductivity | Coordination Compounds |
 |
3 moles of  |
1:3 electrolyte |  |
 |
2 moles of |
1:2 electrolyte |  |
 |
1 mole of |
1:1 electrolyte |  |
 |
1 mole of |
1:1 electrolyte | |
These results, along with the conductivity measurements in solution, was explained if six groups in total, either chloride ions or ammonia molecules or both, remain bonded to the cobalt ion during the reaction and the compounds are formulated where the atoms within the square brackets form a single entity that does not dissociate under the reaction conditions. These are known as coordination compounds.
Postulates of Werner’s Theory
Werner proposed his theory of coordination compounds and the following are the main postulates:
- Metals in coordination compounds have two types of linkages (valences): primary and secondary.
- Negative ions satisfy the primary valences, which are normally ionisable. For metals, it is equal to the oxidation number of metals.
- The secondary valences cannot be ionized. These are generally neutral molecules or negatively charged ions. For a metal, the secondary valence is equal to the coordination number and is fixed.
- The ions/groups bound to the metal by secondary linkages have distinct spatial arrangements that correspond to different coordination numbers.
Coordination Polyhedra
Such spatial arrangements are known as coordination polyhedra in modern formulations. Coordination entities or complexes are the species within the square bracket, and counter ions are the ions outside the square bracket. In the spatial arrangement, primary valency is shown by a dotted line and secondary valency is shown by a solid line.
Limitations of Werner’s theory
- Werner did not explain the colour of the coordination compound, despite explaining some of its properties.
- He was unable to explain coordination compounds' magnetic and optical properties.
- He was unable to answer why the coordination sphere has a definite geometry.
Practice Problems
Q 1. 0.04 moles of 
and 0.08 moles of
are present in 500 mL of a solution X. The number of moles of the precipitate Y and Z that are formed when the solution X is treated with excess silver nitrate and excess barium chloride is respectively:
a. 0.04, 0.08
b. 0.08, 0.04
c. 0.08, 0.08
d. 0.04, 0.04
Answer: When 0.04 moles of dissociates, it will give 0.08 mole of chloride ions.
0.04 moles 0.08 moles
0.08 moles of silver chloride precipitate will result from reaction with silver nitrate.
0.08 moles 0.08 moles
When 0.08 moles of 
dissociates, it will give 0.08 mole of sulphate ions.
0.08 moles 0.08 moles
0.08 moles of barium sulphate precipitate will result from reaction with barium chloride.
0.08 moles 0.08 moles
So, the correct answer is an option (C).
Q 2. 0.05 moles of 
and 0.05 moles of
are present in 200 mL of a solutions A. The number of moles of the precipitate B and C that are formed when the solution A is treated with excess barium chloride and excess silver nitrate is respectively:
a. 0.05, 0.05
b. 0.10, 0.15
c. 0.05, 0.15
d. 0.10, 0.05
Answer: When 0.05 moles of 
dissociates, it will give 0.05 mole of sulphate ions.
0.08 moles 0.08 moles
0.05 moles of barium sulphate precipitate will result from reaction with barium chloride.
0.05 moles 0.05 moles
When 0.05 moles of 
dissociates, it will give 0.15 mole of chloride ions.
0.05 moles 0.15 moles
0.15 moles of silver chloride precipitate will result from reaction with silver nitrate.
0.15 moles 0.15 moles
So, the correct answer is an option (C).
Q 3. A cation exchanger is used to pass a solution containing 5.35 g of 
(molar mass = 267.5 
). The chloride ions in the solution were treated with an excess of 
to yielding 5.74 g of AgCl (molar mass = 143.5 ). The complex formula and primary valency and secondary valencies are
a. 
, 3, 6
b. 
, 3, 6
c. 2, 6
d. 2, 6
Answer: Moles of = 
Moles of AgCl = 
It means if we have 1 mole of , there will be precipitation of 2 moles of AgCl occurs on addition of 
. This indicates two mole of 
are ionisable. Hence, the reaction can be written as
So, the complex formula should be .
Secondary valency corresponds to the coordination number of a compound. So, S.V. = 6
Primary valency corresponds to the oxidation number of a compound. So, P.V. = +3
So, the correct answer is an option (B).
Q 4. The chloride ions in the solution were treated with an excess of 
to yield 1.4 g of 
(molar mass = 233 ). The primary valency and secondary valencies of 1.5 g of 
(molar mass = 249 ).
a. 2,6
b. 1,6
c. 2,4
d. None of these
Answer: Moles of 
= 
Moles of = 
It means if we have 1 mole of 
there will be precipitation of 1 moles of occurs on the addition of 
This indicates 1 mole of 
are ionisable. Hence, the reaction can be written as
So, the complex formula should be 
.
Secondary valency corresponds to the coordination number of a compound. So, S.V. = 6
Primary valency corresponds to the oxidation number of a compound. So, P.V. = +2
So, the correct answer is an option (A).
Frequently Asked Questions
Q 1. What is Werner’s biggest achievement from its theory?
Answer: Werner received the Nobel Prize in Chemistry in 1913 as the first inorganic chemist. He investigated a wide range of complex compounds formed by the reaction of cobalt chloride and ammonia.
Q 2. What are the most important applications of Werner's Theory?
Answer: It accurately predicts the structure of each complex. It explains why a specific metal atom and, more specifically, ligand form different complexes. It also describes the various properties of each complex. It predicts the structure of various complexes containing C.N.
Q 3. Who is the father of coordination chemistry?
Answer: Alfred Werner is the father of coordination chemistry because he provides the basis of modern coordination chemistry with experimental evidence.
Q 4. How did Werner lead to conclude about coordination compounds?
Answer: Werner discovered a variety of cobalt-ammonia chloride forms. These compounds differ in colour and other properties. Although the chemical formula contains three chloride ions per mole, the number of chloride ions that precipitate with 
ions is not always three.
Werner assumed that only ionized chloride ions would precipitate with silver ion. Werner developed the Complex formula and explained the structure of cobalt complexes to distinguish ionized chloride from coordinated chloride.
Related Topics
| Oxidation number of elements in coordination compounds | Organometallic Compounds |
| EAN Rule | Ligands |
| Bonding in coordination compounds |
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