---
title: "Low vs high spin; VBT of [Ni(CO)₄]"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:low-high-spin-ni-co4"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/low-high-spin-ni-co4
site: BCT Engineering
---

# Low vs high spin; VBT of [Ni(CO)₄]

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2069 Bhadra, 2+3 marks
- Video: [High Spin and Low Spin Complexes](https://www.youtube.com/watch?v=hVEQ7-iPPK4) — Chemistry Guru
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/low-high-spin-ni-co4
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

Differentiate between low spin and high spin complexes. Explain the geometry and magnetic behaviour of [Ni(CO)₄]⁰ on the basis of valence bond theory.

## Short answer

Low spin: strong-field ligands, Δo > pairing energy, few unpaired electrons; high spin: weak-field ligands, Δo < pairing energy, maximum unpaired electrons. [Ni(CO)₄]⁰: CO forces pairing to 3d¹⁰, Ni uses sp³ hybridization, tetrahedral, diamagnetic.

## Answer

#### Low spin vs high spin complexes
| Basis | Low spin | High spin |
|---|---|---|
| Ligand field | Strong-field ligands (CN⁻, CO, NH₃) | Weak-field ligands (F⁻, Cl⁻, H₂O) |
| Δo vs pairing energy | Δo > P | Δo < P |
| Unpaired electrons | Few or none (pairing occurs) | Maximum number |
| Orbitals used | Inner (d²sp³, dsp²) | Outer (sp³d², sp³) |
| Magnetism | Weakly paramagnetic or diamagnetic | Strongly paramagnetic |
| Example | [Fe(CN)₆]³⁻ | [FeF₆]³⁻ |

#### [Ni(CO)₄]⁰
Ni is in the 0 state: 3d⁸ 4s². CO is a strong-field ligand, so the two 4s electrons shift into the 3d orbitals, giving 3d¹⁰ (all paired). The empty 4s and three 4p orbitals form four **sp³** hybrid orbitals, which accept the lone pairs from four CO.
```text
3d: [↑↓][↑↓][↑↓][↑↓][↑↓]  4s [ ]  4p [ ][ ][ ] → sp³
```
- **Geometry:** tetrahedral.
- **Magnetism:** no unpaired electron, so **diamagnetic**.

## Check yourself

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

1. Which statement correctly describes low-spin complexes?
   A. They form with weak-field ligands and Δo < P
   B. They have the maximum number of unpaired electrons
   C. They form with strong-field ligands, where the splitting Δo is greater than the pairing energy P
   D. They use outer sp3d2 orbitals
   Answer: C. Strong-field ligands cause pairing (Δo > P), giving few or no unpaired electrons and inner-orbital hybridisation. (Key point: Spin type vs ligand)

2. What happens to the Ni(0) electrons (3d8 4s2) in [Ni(CO)4] because CO is strong-field?
   A. The 4s electrons shift into 3d giving 3d10
   B. The 3d electrons move into 4p
   C. Two 3d electrons are removed
   D. Nothing changes, 3d8 4s2 remains
   Answer: A. CO forces the 4s electrons into 3d, giving a fully paired 3d10 configuration. (Key point: Ni(CO)4 electronic change)

3. Which orbitals hybridise in [Ni(CO)4]?
   A. Two 3d, one 4s and three 4p giving d2sp3
   B. One 3d, one 4s and two 4p giving dsp2
   C. One 4s, three 4p and two 4d giving sp3d2
   D. One 4s and three 4p giving four sp3
   Answer: D. With 3d full, the empty 4s and three 4p form four sp3 hybrids that accept CO lone pairs. (Key point: Hybridisation in Ni(CO)4)

4. What are the geometry and magnetic behaviour of [Ni(CO)4]?
   A. Square planar and paramagnetic
   B. Tetrahedral and diamagnetic
   C. Tetrahedral and paramagnetic
   D. Octahedral and diamagnetic
   Answer: B. sp3 gives a tetrahedral shape, and 3d10 has no unpaired electron, so it is diamagnetic. (Key point: Geometry and magnetism)

## Related in Engineering Chemistry

- [Tetrahedral and square planar complexes; [Ni(NH₃)₄]²⁺ VBT](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atetra-square-vbt-ni-nh3) — Engineering Chemistry, chapter 6
- [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×
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- [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
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- [Inner/outer orbital complexes; VBT of [Fe(CN)₆]³⁻](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ainner-outer-vbt-fecn6) — Engineering Chemistry, chapter 6

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