---
title: "Werner's postulates; IUPAC names [partly illegible]"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:werner-postulates-iupac"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/werner-postulates-iupac
site: BCT Engineering
---

# Werner's postulates; IUPAC names [partly illegible]

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2071 Magh
- 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-postulates-iupac
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

What are the main postulates of Werner's theory? W[?] following complexes.
a) [Cu(NH₃)₄(H₂O)₂]²⁺
b) [Co(H₂O)₂(OH)₄]⁻
c) [Co(NH₃)₆]Cl₃
d) [?]₂[Ni(CN)₄]

## Short answer

Metals have primary valency (ionisable, equals oxidation state) and secondary valency (non-ionisable, equals coordination number, directed in space). IUPAC names: (a) tetraamminediaquacopper(II) ion, (b) diaquatetrahydroxocobaltate(III) ion, (c) hexaamminecobalt(III) chloride, (d) potassium tetracyanonickelate(II).

## Answer

**Reading:** the question is "What are the main postulates of Werner's theory? Write the IUPAC names of the following complexes." Part (d) has an illegible cation; I take it as K₂[Ni(CN)₄].

#### 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.

#### IUPAC names
a) [Cu(NH₃)₄(H₂O)₂]²⁺: **tetraamminediaquacopper(II) ion**
b) [Co(H₂O)₂(OH)₄]⁻: **diaquatetrahydroxocobaltate(III) ion**
c) [Co(NH₃)₆]Cl₃: **hexaamminecobalt(III) chloride**
d) K₂[Ni(CN)₄]: **potassium tetracyanonickelate(II)**

## Check yourself

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

1. In Werner's theory, the primary valency equals
   A. coordination number
   B. ligands
   C. none
   D. oxidation state
   Answer: D. The correct option is: oxidation state. This follows from the answer on primary. (Key point: Primary)

2. The secondary valency is satisfied by
   A. negative ions or neutral molecules
   B. only cations
   C. only electrons
   D. none
   Answer: A. The correct option is: negative ions or neutral molecules. This follows from the answer on secondary. (Key point: Secondary)

3. What is the IUPAC name of [Cu(NH3)4(H2O)2]2+?
   A. diaquatetraamminecopper(II) ion
   B. tetraamminediaquacopper(II) ion
   C. tetraamminediaquacuprate(II)
   D. copper tetraammine
   Answer: B. Alphabetical. (Key point: Name)

4. What is the IUPAC name of [Co(H2O)2(OH)4]-?
   A. diaquatetrahydroxocobalt(III) ion
   B. tetrahydroxodiaquacobaltate(III)
   C. diaquatetrahydroxocobaltate(III) ion
   D. cobaltate hydroxo
   Answer: C. x +4(-1) = -1 so +3. (Key point: Anion)

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