---
title: "VBT for [Fe(CN)₆]⁴⁻: geometry, magnetism"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:vbt-fecn6-4minus"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/vbt-fecn6-4minus
site: BCT Engineering
---

# VBT for [Fe(CN)₆]⁴⁻: geometry, magnetism

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2070 Magh, 3 marks; 2067 Mangsir, 3 marks (asked 2 times)
- Video: [Structure of Co ordination Compound  Fe CN6 4  on Basis of VBT - Co-Ordination Compounds](https://www.youtube.com/watch?v=1vI4V0BOzY0) — 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-fecn6-4minus
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

Explain the formation of [Fe(CN)₆]⁴⁻ complex on the basis of VBT. Also predict its geometry and magnetic behavior.

## Short answer

CN- as a strong-field ligand pairs the 3d6 electrons of Fe2+, vacating two 3d orbitals. This yields d2sp3 hybridization, an octahedral inner orbital low-spin complex, and diamagnetic behavior.

## Answer

Fe (Z = 26); in [Fe(CN)₆]⁴⁻ the oxidation state of Fe is +2 (6 × (−1) + x = −4). Fe²⁺ has the configuration 3d⁶.

CN⁻ is a strong-field ligand, so the six 3d electrons pair up in three d orbitals, leaving two 3d orbitals empty. These two 3d, the 4s and three 4p orbitals mix to give **d²sp³** hybrid orbitals, which accept six lone pairs from CN⁻.
```text
Fe²⁺ 3d: [↑↓][↑↓][↑↓][  ][  ]  4s [ ]  4p [ ][ ][ ]
                      \_______ d²sp³ _______/
                      filled by 6 CN⁻ lone pairs
```
- **Geometry:** octahedral (inner orbital, low spin complex).
- **Magnetism:** no unpaired electron, so **diamagnetic**.

## Check yourself

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

1. What is the oxidation state of Fe in [Fe(CN)6]4-?
   A. +2
   B. +3
   C. +4
   D. +6
   Answer: A. The correct option is: +2. This follows from the answer on oxidation state. (Key point: Oxidation state)

2. Why do six 3d electrons pair?
   A. CN- is weak-field
   B. CN- is a strong-field ligand
   C. Fe2+ has no d electrons
   D. Cl- is present
   Answer: B. Pairing leaves two 3d orbitals empty. (Key point: Pairing)

3. What is the hybridisation in [Fe(CN)6]4-?
   A. sp3d2
   B. dsp2
   C. d2sp3
   D. sp3
   Answer: C. Two 3d, 4s, three 4p. (Key point: Hybridisation)

4. What is the magnetic behaviour of [Fe(CN)6]4-?
   A. paramagnetic with 4 unpaired
   B. paramagnetic with 1 unpaired
   C. ferromagnetic
   D. diamagnetic
   Answer: D. No unpaired electrons. (Key point: Magnetism)

## Related in Engineering Chemistry

- [VBT for [Fe(CN)₆]³⁻ and [FeF₆]³⁻ magnetism](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avbt-fecn6-fef6) — Engineering Chemistry, chapter 6
- [VBT formation of [Ni(CO)₄] and [Fe(CN)₆]⁴⁻](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avbt-nico4-fecn6-4) — Engineering Chemistry, chapter 6
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- [VBT for [Ni(NH₃)₆]²⁺: geometry and magnetism](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avbt-nih36) — Engineering Chemistry, chapter 6, asked 2×
- [VBT: magnetism of [Fe(CN)₆]⁴⁻/³⁻ and [Ni(CO)₄]](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avbt-facts-fecn6-nico4) — Engineering Chemistry, chapter 6