---
title: "VBT for [Fe(CN)₆]³⁻ and [FeF₆]³⁻ magnetism"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:vbt-fecn6-fef6"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/vbt-fecn6-fef6
site: BCT Engineering
---

# VBT for [Fe(CN)₆]³⁻ and [FeF₆]³⁻ magnetism

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2073 Magh, 2+2+1 marks
- Video: [Structure of Co ordination Compound  FeF63  Basis of VBT - Co-Ordination Compounds](https://www.youtube.com/watch?v=KWfx80OwoEc) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/vbt-fecn6-fef6
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

Explain the formation of [Fe(CN)₆]³⁻ and [FeF₆]³⁻ on the basis of Valence Bond Theory and also predict their magnetic property.

## Short answer

[Fe(CN)6]3-: Fe3+ (3d5) with strong CN- pair electrons, leaving one unpaired, d2sp3 (inner orbital) octahedral, paramagnetic (μ=1.73 BM). [FeF6]3-: weak F- leaves five unpaired, sp3d2 (outer orbital) octahedral, paramagnetic (μ=5.92 BM).

## Answer

#### [Fe(CN)₆]³⁻
Fe³⁺ is 3d⁵. CN⁻ is a strong-field ligand, so the electrons pair up, leaving one unpaired electron and two empty 3d orbitals. d²sp³ hybridisation (inner orbital) gives an **octahedral** complex.
```text
3d: [↑↓][↑↓][↑ ][  ][  ]  4s 4p  → d²sp³
```
One unpaired electron: **paramagnetic**, μ = √3 = 1.73 BM (low spin).

#### [FeF₆]³⁻
F⁻ is a weak-field ligand, so the 3d⁵ electrons remain unpaired. The outer 4s, 4p and two 4d orbitals form sp³d² hybrids (outer orbital), also **octahedral**.
```text
3d: [↑ ][↑ ][↑ ][↑ ][↑ ]  4s 4p 4d  → sp³d²
```
Five unpaired electrons: **paramagnetic**, μ = √35 = 5.92 BM (high spin).

Both are paramagnetic, but [FeF₆]³⁻ is far more strongly paramagnetic.

## Check yourself

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

1. How many unpaired electrons does [Fe(CN)6]3- have?
   A. 1
   B. 5
   C. 0
   D. 3
   Answer: A. The correct option is: 1. This follows from the answer on cn-. (Key point: CN-)

2. Which hybridisation is used in [FeF6]3-?
   A. d2sp3
   B. sp3d2
   C. dsp2
   D. sp3
   Answer: B. The correct option is: sp3d2. This follows from the answer on f-. (Key point: F-)

3. How many unpaired electrons does [FeF6]3- have?
   A. 1
   B. 0
   C. 5
   D. 3
   Answer: C. The correct option is: 5. This follows from the answer on f-. (Key point: F-)

4. Which of [Fe(CN)6]3- and [FeF6]3- is more strongly paramagnetic?
   A. [Fe(CN)6]3-
   B. Equal
   C. Neither
   D. [FeF6]3-
   Answer: D. 5.92 vs 1.73. (Key point: Compare)

## Related in Engineering Chemistry

- [VBT for [Fe(CN)₆]⁴⁻: geometry, magnetism](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avbt-fecn6-4minus) — Engineering Chemistry, chapter 6, asked 2×
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