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

# Formulae of named complexes

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2068 Bhadra, 2 marks
- Video: [Naming Coordination Compounds - Chemistry](https://www.youtube.com/watch?v=yeNtqjE0dJw) — The Organic Chemistry Tutor
- 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-complexes
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

Write the formulae of following:
i) Potassium hexacyanoferrate (III)
ii) Trioxalatoaluminate (III) ion
iii) Tris(ethylenediamine) chromium (III) chloride
iv) Bis(benzene) chromium(0)

## Short answer

i) K₃[Fe(CN)₆] ii) [Al(C₂O₄)₃]³⁻ iii) [Cr(en)₃]Cl₃ iv) [Cr(C₆H₆)₂].

## Answer

i) Potassium hexacyanoferrate(III): **K₃[Fe(CN)₆]**
ii) Trioxalatoaluminate(III) ion: **[Al(C₂O₄)₃]³⁻**
iii) Tris(ethylenediamine)chromium(III) chloride: **[Cr(en)₃]Cl₃**
iv) Bis(benzene)chromium(0): **[Cr(C₆H₆)₂]**

Charges are checked: (i) 3(+1) + (+3) + 6(−1) = 0; (ii) +3 + 3(−2) = −3; (iii) +3 + 3(−1) = 0; (iv) 0 + 0 = 0.

## Check yourself

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

1. What is the formula of potassium hexacyanoferrate(III)?
   A. K4[Fe(CN)6]
   B. K3[Fe(CN)6]
   C. K2[Fe(CN)6]
   D. K3[Fe(CN)3]
   Answer: B. Fe3+ with six CN- gives [Fe(CN)6]3-, balanced by 3 K+. (Key point: Formula (i))

2. What is the formula of trioxalatoaluminate(III) ion?
   A. [Al(C2O4)3]3+
   B. [Al(C2O4)]3-
   C. [Al(C2O4)3]-
   D. [Al(C2O4)3]3-
   Answer: D. +3 + 3(-2) = -3. (Key point: Formula (ii))

3. What is the formula of tris(ethylenediamine)chromium(III) chloride?
   A. [Cr(en)3]Cl3
   B. [Cr(en)3]Cl2
   C. [Cr(en)]Cl3
   D. [Cr(en)3Cl3]
   Answer: A. en is neutral, so the cation is 3+, needing three Cl-. (Key point: Formula (iii))

4. What is bis(benzene)chromium(0)?
   A. [Cr(C6H6)2]2+
   B. [Cr(C6H6)]
   C. [Cr(C6H6)2]
   D. K[Cr(C6H6)2]
   Answer: C. Cr(0) with two neutral benzene ligands is a neutral molecule. (Key point: Formula (iv))

## Related in Engineering Chemistry

- [Formulae of six named complexes](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awrite-formulae-2068m) — Engineering Chemistry, chapter 6
- [Formulae of six named complexes](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awrite-formulae-2067c) — Engineering Chemistry, chapter 6