---
title: "d-d transition and origin of colour"
subject: "Engineering Chemistry"
chapter: 5
key: "semester_2/engineering_chemistry:d-d-transition"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/d-d-transition
site: BCT Engineering
---

# d-d transition and origin of colour

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

- Asked in: 2072 Magh, 5 marks
- Video: [Why do Transition Metals Form Coloured Compounds?](https://www.youtube.com/watch?v=mxBD-1aTB6E) — Chemistry Student
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/d-d-transition
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-5

## Question

What is meant by d to d transition? How does the color of the compound originate? Explain with d to d transition.

## Short answer

A d–d transition is an electron jump from the lower t2g to higher eg set of a ligand-field split d orbital, absorbing light. Colour arises because the absorbed wavelength matches Δo; the transmitted complementary colour is seen, e.g., [Ti(H2O)6]3+ absorbs green-yellow and appears violet.

## Answer

**d–d transition:** in a transition metal complex the five d orbitals are split by the ligand field into two sets of different energy. The jump of an electron from the lower set to the higher set by absorbing light is called a d–d transition.

#### Splitting of d orbitals (octahedral)
```text
        ___ ___   e_g (dz², dx²−y²)
         ↑   Δo
        ___ ___ ___  t₂g (dxy, dyz, dxz)
```
In an isolated ion the five d orbitals have equal energy. When ligands approach, the repulsion raises the energy of e_g orbitals (which point at the ligands) more than the t₂g orbitals, giving the gap Δo.

#### Origin of colour
1. White light falls on the complex.
2. If Δo corresponds to a visible wavelength, an electron in t₂g absorbs that part of the light and moves to e_g.
3. The remaining wavelengths are transmitted, and the eye sees the **complementary colour**.

Example: [Ti(H₂O)₆]³⁺ (d¹) absorbs green-yellow (≈ 500 nm) and appears violet; [Cu(H₂O)₄]²⁺ (d⁹) absorbs red-orange and appears blue.

Ions with d⁰ (Sc³⁺, Ti⁴⁺) or d¹⁰ (Zn²⁺, Cu⁺) cannot show d–d transitions, so they are colourless. The colour depends on the metal, its oxidation state and the ligands, since these change Δo.

## Check yourself

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

1. What is a d-d transition?
   A. Jump of an electron from an s orbital to a p orbital
   B. Transfer of an electron from ligand to metal
   C. Pairing of electrons in the same d orbital
   D. Jump of an electron from the lower set of split d orbitals to the higher set on absorbing light
   Answer: D. Ligand field splits the d orbitals into two sets; absorption of light promotes an electron between them. (Key point: Definition of d-d transition)

2. In an octahedral complex, which d orbitals are raised higher in energy and why?
   A. e_g, because they point directly at the ligands and feel more repulsion
   B. t2g, because they point at the ligands
   C. e_g, because they lie between the ligands
   D. Neither; all five stay equal in energy
   Answer: A. e_g orbitals (dz2, dx2-y2) point at the ligands, so repulsion raises them more, giving the gap Delta o. (Key point: Octahedral splitting)

3. Why does a complex that absorbs green-yellow light appear violet?
   A. Violet light is absorbed and re-emitted
   B. The unabsorbed wavelengths are transmitted and the eye sees the complementary colour
   C. Violet is the colour of the metal atom itself
   D. The complex absorbs all colours except green-yellow
   Answer: B. The remaining transmitted light appears as the complementary colour of the absorbed light. (Key point: Why a colour is seen)

4. Which ions cannot show d-d transitions and are colourless?
   A. d5 ions such as Mn2+
   B. d1 ions such as Ti3+
   C. d0 and d10 ions such as Sc3+ and Zn2+
   D. d9 ions such as Cu2+
   Answer: C. d0 has no electron to excite and d10 has no vacant d orbital to accept one. (Key point: Colourless ions)

## Related in Engineering Chemistry

- [Electrode potential origin; Zn reduction potential](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aelectrode-potential-zn) — Engineering Chemistry, chapter 1
- [Electrode potential origin; Zn-Cu cell notation](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aelectrode-potential-zn-cu) — Engineering Chemistry, chapter 1

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