---
title: "Complex formation by transition elements"
subject: "Engineering Chemistry"
chapter: 5
key: "semester_2/engineering_chemistry:complex-formation-tm"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/complex-formation-tm
site: BCT Engineering
---

# Complex formation by transition elements

Engineering Chemistry · Chapter 5: 3-d Transition Elements and their Applications

- Asked in: 2075 Baisakh, 2 marks; 2073 Bhadra, 2.5 marks; 2073 Magh, 2 marks; 2072 Asoj, 2.5 marks; 2071 Magh, 2.5 marks; 2070 Magh, 2 marks; 2069 Bhadra; 2068 Bhadra, 3 marks; 2068 Magh, 2.5 marks; 2067 Mangsir, 3 marks; 2067 Chaitra, 3 marks (asked 11 times)
- Video: [Transition metal complex ions](https://www.youtube.com/watch?v=BS4Rmr7G28s) — Allery Chemistry
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/complex-formation-tm
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-5

## Question

Why do transition elements form significant number of complexes?

## Short answer

Transition metals form many complexes because of their small size, high charge density, vacant d orbitals accepting lone pairs, variable oxidation states, and ligand field stabilisation. Examples: [Fe(CN)6]4- and [Cu(NH3)4]2+.

## Answer

Transition elements form a large number of stable complexes because:

1. **Small size and high charge density:** the ions are small and carry a high positive charge, so they strongly attract ligands (lone-pair donors).
2. **Vacant d orbitals of suitable energy:** empty (n−1)d, ns and np orbitals are available to accept lone pairs from ligands and form coordinate (dative) bonds.
3. **Variable oxidation states** allow different charges and so a variety of complexes.
4. **Ligand field stabilisation:** the d-electrons gain extra stabilisation energy in the field of ligands.

Examples: [Fe(CN)₆]⁴⁻, [Cu(NH₃)₄]²⁺, [Ni(CO)₄], [Co(NH₃)₆]³⁺, [Ag(NH₃)₂]⁺. In contrast, s-block metals have large ions with low charge density and no d orbitals, so they form few complexes.

## Check yourself

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

1. Why does small size and high charge density help complex formation?
   A. Ions strongly attract ligand lone pairs
   B. They repel ligands
   C. They increase atomic mass
   D. They remove d electrons
   Answer: A. High charge density gives strong attraction to ligands. (Key point: Size and charge)

2. What do vacant d, s and p orbitals of suitable energy allow?
   A. They donate electrons to ligands
   B. They accept lone pairs from ligands to form coordinate bonds
   C. They make ions colourless
   D. They stop bonding
   Answer: B. Empty orbitals accept electron pairs. (Key point: Vacant d orbitals)

3. Why do s-block metals form few complexes?
   A. Small ions with high charge
   B. They have many vacant d orbitals
   C. Large ions with low charge density and no d orbitals
   D. They have variable oxidation states
   Answer: C. These reasons contrast with transition metals. (Key point: s-block comparison)

4. What extra stabilisation do d electrons gain in a complex?
   A. Lattice energy
   B. Ionisation energy
   C. Hydration of ligand
   D. Ligand field stabilisation energy
   Answer: D. d electrons gain energy in the ligand field. (Key point: Ligand field)

## Related in Engineering Chemistry

- [Transition elements; oxidation states of 3d; alloys](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-ox-alloy) — Engineering Chemistry, chapter 5
- [Why d-block elements are called transition](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awhy-transition-elements) — Engineering Chemistry, chapter 5, asked 2×
- [Transition elements; first series configuration](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awhy-transition-elements-typical) — Engineering Chemistry, chapter 5
- [Transition elements; configuration; paramagnetism](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-config-paramagnetism) — Engineering Chemistry, chapter 5
- [Transition elements; non-typical 3d element](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-not-typical) — Engineering Chemistry, chapter 5
- [General outer electronic configuration of transition elements](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-general-config) — Engineering Chemistry, chapter 5

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