---
title: "Magnetic difference of [Co(NH₃)₆]³⁺ and [CoF₆]³⁻"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:co-nh3-cof6"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/co-nh3-cof6
site: BCT Engineering
---

# Magnetic difference of [Co(NH₃)₆]³⁺ and [CoF₆]³⁻

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2075 Baisakh, 2.5+2.5 marks
- Video: [[CoF6]3- is Paramagnetic and [Co(NH3)6]3+  is Diamagnetic why? Valance bond theory Coordination comp](https://www.youtube.com/watch?v=Vhy_ZOcJPjA) — Vicky Science Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/co-nh3-cof6
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

[Co(NH₃)₆]³⁺ and [CoF₆]³⁻ are both octahedral but show marked difference in their magnetic properties. Explain.

## Short answer

[Co(NH3)6]3+ is low-spin d2sp3, octahedral, diamagnetic. [CoF6]3- is high-spin sp3d2, octahedral, paramagnetic with four unpaired electrons (4.90 BM). The difference is due to strong-field NH3 versus weak-field F-.

## Answer

Co³⁺ has the configuration 3d⁶ in both. The difference comes from the strength of the ligands.

#### [Co(NH₃)₆]³⁺
NH₃ is a strong-field ligand. The 3d electrons pair up, leaving two 3d orbitals empty. d²sp³ hybridisation gives an octahedral **inner orbital (low spin)** complex.
```text
3d: [↑↓][↑↓][↑↓][  ][  ]  d²sp³
```
No unpaired electrons: **diamagnetic** (μ = 0).

#### [CoF₆]³⁻
F⁻ is a weak-field ligand and cannot force pairing. The 3d⁶ electrons remain with four unpaired electrons and the outer 4s, 4p and 4d orbitals are used (sp³d²). It is an octahedral **outer orbital (high spin)** complex.
```text
3d: [↑↓][↑ ][↑ ][↑ ][↑ ]  4s 4p 4d: sp³d²
```
Four unpaired electrons: **paramagnetic**, μ = √(4×6) = 4.90 BM.

## Check yourself

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

1. What is the d configuration of Co3+ in both complexes?
   A. 3d6
   B. 3d8
   C. 3d7
   D. 3d9
   Answer: A. The correct option is: 3d6. This follows from the answer on common point. (Key point: Common point)

2. How is [Co(NH3)6]3+ best described?
   A. paramagnetic high spin
   B. diamagnetic, low spin d2sp3
   C. paramagnetic low spin
   D. tetrahedral
   Answer: B. Strong field. (Key point: NH3)

3. What does [CoF6]3- have?
   A. no unpaired electrons
   B. two unpaired electrons
   C. four unpaired electrons, sp3d2
   D. d2sp3
   Answer: C. Weak-field F-. (Key point: F-)

4. What is the spin-only magnetic moment of [CoF6]3-?
   A. 0
   B. 2.83 BM
   C. 5.92 BM
   D. 4.90 BM
   Answer: D. The correct option is: 4.90 BM. This follows from the answer on moment. (Key point: Moment)

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