---
title: "Principal and auxiliary valencies; IUPAC names"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:primary-secondary-valency"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/primary-secondary-valency
site: BCT Engineering
---

# Principal and auxiliary valencies; IUPAC names

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2073 Magh, 3+2 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/primary-secondary-valency
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

What are principle and auxiliary valencies of the metal in the complex compounds? Illustrate with suitable example. Write the IUPAC names of
a) Na[Ag(CN)₂]
b) [Co(NH₃)₄H₂O.Cl]Cl₂
c) [Cr(en)₃]Cl₃
d) K₄[Mn(Cl)₆]

## Short answer

Principal valency is the ionizable oxidation state satisfied by ions outside the coordination sphere; auxiliary valency is the non-ionizable coordination number, directional, satisfied by ligands directly bonded. Example: [Co(NH3)6]Cl3 has principal valency 3 and auxiliary 6. IUPAC names: a) sodium dicyanoargentate(I), b) tetraammineaquachlorocobalt(III) chloride, c) tris(ethylenediamine)chromium(III) chloride, d) potassium hexachloromanganate(II).

## Answer

#### Principal (primary) and auxiliary (secondary) valencies
- **Principal valency:** the ionisable valency of the metal, equal to its oxidation state, satisfied by negative ions. It is shown by dotted lines.
- **Auxiliary valency:** the non-ionisable valency, equal to the coordination number, satisfied by ligands (neutral or negative) attached directly to the metal. It is shown by solid lines and has a fixed direction in space.

Example: in [Co(NH₃)₆]Cl₃, the principal valency is 3 (satisfied by 3 Cl⁻ outside the bracket) and the auxiliary valency is 6 (satisfied by 6 NH₃). In [Co(NH₃)₅Cl]Cl₂, Cl⁻ inside satisfies both valencies.

#### IUPAC names
a) Na[Ag(CN)₂]: **sodium dicyanoargentate(I)**
b) [Co(NH₃)₄(H₂O)Cl]Cl₂: **tetraammineaquachlorocobalt(III) chloride**
c) [Cr(en)₃]Cl₃: **tris(ethylenediamine)chromium(III) chloride**
d) K₄[MnCl₆]: **potassium hexachloromanganate(II)**

## Check yourself

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

1. Principal valency equals
   A. oxidation state, ionisable
   B. coordination number
   C. number of ligands
   D. atomic number
   Answer: A. Satisfied by negative ions. (Key point: Principal valency)

2. Auxiliary valency equals
   A. oxidation state
   B. coordination number, non-ionisable
   C. charge on complex
   D. none
   Answer: B. Satisfied by ligands. (Key point: Auxiliary)

3. In [Co(NH3)6]Cl3, what is the principal valency?
   A. 6
   B. 9
   C. 3
   D. 0
   Answer: C. The correct option is: 3. This follows from the answer on example. (Key point: Example)

4. What is the IUPAC name of Na[Ag(CN)2]?
   A. sodium dicyanosilver(I)
   B. sodium dicyanoargentate(II)
   C. silver dicyanosodium
   D. sodium dicyanoargentate(I)
   Answer: D. Anionic Ag uses argentate. (Key point: Name)

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