---
title: "VBT formation of [Ni(CO)₄] and [Fe(CN)₆]⁴⁻"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:vbt-nico4-fecn6-4"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/vbt-nico4-fecn6-4
site: BCT Engineering
---

# VBT formation of [Ni(CO)₄] and [Fe(CN)₆]⁴⁻

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2068 Magh, 2.5+2.5 marks
- Video: [[Ni(CO)4] is tetrahedral and [Ni(CN)4]2-  is Square Planar ?Why coordinate compound VBT](https://www.youtube.com/watch?v=La_CXLjcsaw) — 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/vbt-nico4-fecn6-4
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

On the basis of VBT of co-ordination compounds, explain the formation of [Ni(CO)₄]⁰ and [Fe(CN)₆]⁴⁻.

## Short answer

[Ni(CO)₄]⁰: Ni(0) 3d¹⁰, sp³ hybrids accept CO lone pairs, tetrahedral, diamagnetic. [Fe(CN)₆]⁴⁻: Fe²⁺ 3d⁶ pairs, d²sp³ hybrids accept CN⁻, octahedral, diamagnetic.

## Answer

#### [Ni(CO)₄]⁰
Ni (0) is 3d⁸ 4s². CO is a strong-field ligand, so the 4s electrons move into 3d, giving 3d¹⁰. The vacant 4s and three 4p orbitals form four sp³ hybrid orbitals, which accept four CO lone pairs.
```text
3d: [↑↓][↑↓][↑↓][↑↓][↑↓]  4s [ ]  4p [ ][ ][ ] → sp³
```
Shape: **tetrahedral**; no unpaired electron, so diamagnetic.

#### [Fe(CN)₆]⁴⁻
Fe²⁺ is 3d⁶. CN⁻ is a strong-field ligand, so the six electrons pair in three 3d orbitals and two 3d orbitals are vacant. Two 3d, one 4s and three 4p orbitals form six d²sp³ hybrids that accept six CN⁻ lone pairs.
```text
3d: [↑↓][↑↓][↑↓][  ][  ]  4s [ ]  4p [ ][ ][ ] → d²sp³
```
Shape: **octahedral** (inner orbital); no unpaired electron, so diamagnetic.

## Check yourself

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

1. How do the electrons rearrange in Ni(0) when forming [Ni(CO)4]?
   A. 3d electrons pair and 4s stays filled
   B. 4s electrons move into 3d giving 3d10, then empty 4s and 4p form sp3
   C. d2sp3 forms using two 3d orbitals
   D. sp3d2 forms using 4d orbitals
   Answer: B. CO strong-field pushes the 4s pair into 3d, leaving 4s/4p vacant for sp3. (Key point: Ni(CO)4 sp3)

2. How many 3d orbitals are vacated in [Fe(CN)6]4-?
   A. None
   B. Four
   C. One
   D. Two
   Answer: D. Six d electrons pair in three orbitals, leaving two 3d vacant for d2sp3. (Key point: Fe2+ in cyanide complex)

3. Which hybridisation occurs in [Fe(CN)6]4-?
   A. d2sp3 (two 3d, one 4s, three 4p)
   B. sp3d2
   C. sp3
   D. dsp2
   Answer: A. Strong CN- forms an inner-orbital octahedral complex. (Key point: Hybridisation in [Fe(CN)6]4-)

4. What are the shapes and magnetism of [Ni(CO)4] and [Fe(CN)6]4-?
   A. Square planar and octahedral; both paramagnetic
   B. Tetrahedral and octahedral; both paramagnetic
   C. Tetrahedral and octahedral; both diamagnetic
   D. Octahedral and tetrahedral; diamagnetic and paramagnetic
   Answer: C. Neither complex has unpaired electrons. (Key point: Shape and magnetism)

## Related in Engineering Chemistry

- [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
- [VBT: [Ni(CN)₄]²⁻ and [Ni(CO)₄]](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avbt-nicn4-nico4) — Engineering Chemistry, chapter 6
- [VBT for [Ni(CN)₄]²⁻ and its magnetism](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avbt-nicn4) — Engineering Chemistry, chapter 6
- [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×
- [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
- [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

## Related in other subjects

- [Define co-ordination in protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adefine-coordination) — Switchgear and Protection, chapter 1