---
title: "Formulae of six named complexes"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:write-formulae-2068m"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/write-formulae-2068m
site: BCT Engineering
---

# Formulae of six named complexes

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2068 Magh, 3 marks
- Video: [Coordination Compounds: Geometry and Nomenclature](https://www.youtube.com/watch?v=Hs5JBjX51dc) — Professor Dave Explains
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/write-formulae-2068m
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

Write the formulae of the following co-ordination compounds.
a. Sodium hexacyanoferrate (II)
b. Hexaamminecobalt (III) chloride
c. Bromochlorotetraamminecobalt (III) sulfate
d. Hexafluoroaluminate (III) ion
e. Potassium tetrachlorozincate (II)
f. Iron (II) tetrabromoplatinate (II)

## Short answer

a. Na₄[Fe(CN)₆] b. [Co(NH₃)₆]Cl₃ c. [Co(NH₃)₄BrCl]₂SO₄ d. [AlF₆]³⁻ e. K₂[ZnCl₄] f. Fe[PtBr₄].

## Answer

a. Sodium hexacyanoferrate(II): **Na₄[Fe(CN)₆]**
b. Hexaamminecobalt(III) chloride: **[Co(NH₃)₆]Cl₃**
c. Bromochlorotetraamminecobalt(III) sulfate: **[Co(NH₃)₄BrCl]₂SO₄** (the cation is +1: Co³⁺ + Br⁻ + Cl⁻)
d. Hexafluoroaluminate(III) ion: **[AlF₆]³⁻**
e. Potassium tetrachlorozincate(II): **K₂[ZnCl₄]**
f. Iron(II) tetrabromoplatinate(II): **Fe[PtBr₄]**

In the formulae, the cation is written first, ligands inside the bracket follow the metal symbol, and the total charge is balanced.

## Check yourself

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

1. What is sodium hexacyanoferrate(II)?
   A. Na3[Fe(CN)6]
   B. Na2[Fe(CN)6]
   C. Na4[Fe(CN)6]
   D. Na[Fe(CN)6]
   Answer: C. Fe2+ + 6CN- gives 4-, so four Na+. (Key point: Formula a)

2. What is hexaamminecobalt(III) chloride?
   A. [Co(NH3)6]Cl3
   B. [Co(NH3)6]Cl2
   C. [Co(NH3)6]Cl
   D. [Co(NH3)3]Cl6
   Answer: A. Complex cation is 3+, needing three Cl-. (Key point: Formula b)

3. What is bromochlorotetraamminecobalt(III) sulfate?
   A. [Co(NH3)4BrCl]SO4
   B. [Co(NH3)4BrCl]3(SO4)2
   C. [Co(NH3)4BrCl]Cl
   D. [Co(NH3)4BrCl]2SO4
   Answer: D. The cation is +1 (Co3+ + Br- + Cl-), so two cations balance one SO4 2-. (Key point: Formula c)

4. Which pair of formulae is correct for potassium tetrachlorozincate(II) and iron(II) tetrabromoplatinate(II)?
   A. K[ZnCl4] and Fe2[PtBr4]
   B. K2[ZnCl4] and Fe[PtBr4]
   C. K2[ZnCl4] and Fe2[PtBr4]
   D. K4[ZnCl4] and Fe[PtBr4]
   Answer: B. Zn2+ + 4Cl- gives 2-, so 2 K+; Pt2+ + 4Br- gives 2-, balancing Fe2+. (Key point: Formula e and f)

## Related in Engineering Chemistry

- [Formulae of six named complexes](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awrite-formulae-2067c) — Engineering Chemistry, chapter 6
- [Formulae of named complexes](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awrite-formulae-complexes) — Engineering Chemistry, chapter 6
- [Define co-ordination number with example](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Adefine-coordination-number) — Engineering Chemistry, chapter 6
- [Complex ion and ligands in [Co(NH₃)₆]Cl₃](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aidentify-complex-ion-ligands) — Engineering Chemistry, chapter 6

## Related in other subjects

- [Define co-ordination in protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adefine-coordination) — Switchgear and Protection, chapter 1