---
title: "Complex salt; main postulates of Werner's theory"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:complex-salt-werner-postulates"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/complex-salt-werner-postulates
site: BCT Engineering
---

# Complex salt; main postulates of Werner's theory

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2067 Mangsir, 1+4 marks
- Video: [Coordination Compounds | Class 12 Chemistry | Werners Theory | iWiz Divya](https://www.youtube.com/watch?v=c_tn9G0rFNI) — Iwiz Education
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/complex-salt-werner-postulates
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

What is complex salt? Give the main postulates of Werner's coordination theory.

## Short answer

A complex salt contains a complex ion that keeps its identity in solution. Werner’s theory: metals have primary (ionisable, oxidation state) and secondary (coordination number) valencies; secondary valencies are directed in space, giving definite geometry; ligands inside the coordination sphere satisfy secondary valency and do not ionise.

## Answer

#### Complex salt
A complex salt contains a complex ion, formed by a central metal ion bound to ligands by coordinate bonds. The complex ion keeps its identity in solution and does not give the usual tests of its metal and ligands. Example: K₄[Fe(CN)₆], which gives K⁺ and [Fe(CN)₆]⁴⁻ (not Fe²⁺ or CN⁻) in water.

#### Main postulates of Werner's theory
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.

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.

## Check yourself

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

1. What is characteristic of a complex salt in solution?
   A. It splits fully into simple ions
   B. The complex ion keeps its identity and does not give the usual tests of its metal or ligands
   C. It forms no ions
   D. It is always insoluble
   Answer: B. K4[Fe(CN)6] gives K+ and [Fe(CN)6]4-, not Fe2+ or CN-. (Key point: Complex salt)

2. In Werner's theory, which valency is ionisable and equals the oxidation state?
   A. Secondary valency
   B. Coordination valency
   C. Covalent valency
   D. Primary valency
   Answer: D. Primary valency is ionisable; secondary equals coordination number and is non-ionisable. (Key point: Two valencies)

3. What satisfies secondary valency?
   A. Negative ions or neutral molecules (ligands) directed in space
   B. Only outside ions
   C. Only electrons
   D. Only cations
   Answer: A. Secondary valencies are directional and filled by ligands inside the sphere. (Key point: Secondary valency)

4. In [Co(NH3)6]Cl3, which statement is correct?
   A. Only 1 Cl- precipitates
   B. All 6 NH3 ionise
   C. 3 Cl- are outside and precipitate with AgNO3; 6 NH3 are inside the octahedral sphere
   D. Primary valency is 6
   Answer: C. Primary valency 3 is met by outer Cl-; secondary valency 6 by NH3. (Key point: CoCl3.6NH3)

## Related in Engineering Chemistry

- [Double salt vs complex salt; Werner's theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Adouble-complex-werner) — Engineering Chemistry, chapter 6, asked 5×
- [Werner's postulates; IUPAC names [partly illegible]](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awerner-postulates-iupac) — Engineering Chemistry, chapter 6
- [Werner's theory and structure of complexes](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awerner-theory) — Engineering Chemistry, chapter 6