---
title: "Werner's theory and structure of complexes"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:werner-theory"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/werner-theory
site: BCT Engineering
---

# Werner's theory and structure of complexes

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2072 Asoj, 3 marks
- Video: [Werner's theory of coordination compounds| Coordination Compounds | Chemistry | Khan Academy](https://www.youtube.com/watch?v=igyKR_NwD78) — Khan Academy India - English
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/werner-theory
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

How does Werner's theory explain the structures of complex compounds?

## Short answer

Werner proposed two valencies: primary (ionizable oxidation state) and secondary (non-ionizable coordination number, fixed in direction). Secondary valency determines geometry: CN 6 octahedral, CN 4 square planar or tetrahedral. Ligands in the coordination sphere do not ionize. Example: [Co(NH3)6]Cl3 has primary 3, secondary 6, octahedral.

## Answer

Werner's theory (1893) explains the structure of complexes through these postulates:

1. A metal in a complex shows **two kinds of valency**: **primary (ionisable)** valency, which is equal to the oxidation state and is satisfied by negative ions; and **secondary (non-ionisable)** valency, which is equal to the coordination number and is satisfied by negative ions or neutral molecules (ligands).
2. Every metal tries to satisfy both its primary and secondary valencies.
3. The secondary valencies are directed in space, so each complex has a definite geometry: coordination number 6 gives an octahedral complex, and 4 gives a tetrahedral or square planar complex.
4. Ligands that satisfy the secondary valency are attached directly to the metal inside the **coordination sphere** (written in square brackets) and do not ionise; the primary valency is satisfied by ions outside the sphere that ionise in water.

#### Structures predicted
```text
 Octahedral (CN 6)      Square planar (CN 4)
        L                  L         L
        |                    \     /
    L --M-- L                  M
       /|\                   /     \
      L | L                 L         L
        L
```
Example: CoCl₃·6NH₃, formula [Co(NH₃)₆]Cl₃. Primary valency 3 is satisfied by 3 Cl⁻ (outside), secondary valency 6 by 6 NH₃ (inside, octahedral). All 3 Cl⁻ precipitate with AgNO₃. Werner supported this by conductivity and AgNO₃ precipitation experiments, and by isolating the predicted cis/trans isomers of [Co(NH₃)₄Cl₂]⁺.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which two kinds of valency does Werner's theory assign to a metal?
   A. primary (ionisable) and secondary (non-ionisable)
   B. both ionisable
   C. both non-ionisable
   D. covalent and ionic
   Answer: A. The correct option is: primary (ionisable) and secondary (non-ionisable). This follows from the answer on valencies. (Key point: Valencies)

2. What is true of secondary valencies?
   A. have no direction
   B. are directed in space giving definite geometry
   C. are ionisable
   D. equal oxidation state
   Answer: B. CN 6 octahedral. (Key point: Direction)

3. What happens to ligands inside the coordination sphere in water?
   A. ionise
   B. precipitate with AgNO3
   C. do not ionise
   D. are outside
   Answer: C. Square brackets. (Key point: Coordination sphere)

4. How many Cl- ions precipitate with AgNO3 from CoCl3.6NH3, [Co(NH3)6]Cl3?
   A. 0
   B. 1
   C. 2
   D. 3
   Answer: D. All outside. (Key point: AgNO3)

## Related in Engineering Chemistry

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