Mini Chemistry
Part of Mini EducationSwitch subject

d-Orbital Splitting and Colour

Learn and apply d-Orbital Splitting and Colour in the published Chemistry course sequence.

  • GCE A-Level H2 Chemistry 9476-2027
On this page

d-Orbital Splitting and Colour: Orientation

Colour questions are not “magic”: ligands split the d orbitals, and electrons absorb visible light to jump between split levels (a d–d transition). This lesson shows the exam-safe explanation and what changes the splitting (ligand, oxidation state, geometry).

This chapter depends on periodic trends, so keep The Periodic Table (A Level) nearby and use the Transition Elements hub for the full sequence.

Definitions (Must Know)

A. Degenerate d orbitals

Orbitals are degenerate when they have the same energy. In an isolated transition-metal ion, the five d orbitals are degenerate.

B. d-orbital splitting

In an octahedral complex, six ligands approach along the coordinate axes. The differently oriented d orbitals experience different repulsions and split into a lower set of three and a higher set of two.

C. d–d transition

A d–d transition occurs when a d electron absorbs a photon whose energy matches the splitting and is promoted from the lower d level to the higher d level.

Detailed Explanations

A. Shape and orientation cause splitting

In an octahedral complex, ligands approach along the x, y and z axes. The d_(x²-y²) and d_z² orbitals point more directly towards those ligands and experience greater repulsion, so they form the higher-energy pair. The d_xy, d_xz and d_yz orbitals point between the axes and form the lower-energy trio.

B. From splitting to observed colour

An electron absorbs a photon when E = hν = Δ E and moves to the higher d level. Removing that wavelength from incident white light means the transmitted or reflected light appears the complementary colour.

C. Explaining a colour change

First identify what changed: ligand, metal oxidation state or complex identity. Then state that the new environment produces a different Δ E, so a different wavelength is absorbed and a different complementary colour is observed. Do not claim that concentration alone changes the splitting: dilution can make a solution paler without changing its hue.

D. Limits of the model

This is the required qualitative d–d-transition explanation. The relative order of ligand field strengths is not required. Avoid memorising an unsupported ligand series or predicting an exact colour without supplied observations.

Octahedral d-orbital splitting and complex colourSix ligands approach a metal ion along the coordinate axes. The two d orbitals directed towards the ligands rise above the three directed between the axes. Visible light matching the energy gap is absorbed and the complementary colour is observed.Exam map: octahedral approach → d-orbital splitting → colour1) Six ligands approach along the axesMOrbitals pointing at ligands experience greater repulsion.2) Five d orbitals split into two levelshigher: d(x²−y²), d(z²)lower: d(xy), d(xz), d(yz)photon: E = ΔEA d electron can absorb light and move to the higher level.3) Absorption determines the observed complementary colourPhoton relationship: E = hν = hc/λ. A larger ΔE requires a higher-frequency, shorter-wavelength photon.The absorbed wavelength is removed from white light; the complementary colour is seen.Colour-change sentence: a new metal oxidation state or ligand environment changes ΔE,so a different wavelength is absorbed and a different complementary colour is observed.
Follow the assessed causal chain: ligand orientation produces d-orbital splitting, the matching visible wavelength is absorbed, and the complementary colour is observed.

Worked Examples

Modelled example 1

Why Cu²⁺ Compounds Are Coloured

Core

Problem

Explain why Cu²⁺ compounds are often coloured.
Study the worked solution

  1. Identify the electron configuration

    Method

    State Cu²⁺ is 3d⁹.

    Reason

    Its d subshell is incomplete.

    Working

    Cu²⁺:3d^9.

  2. Split and excite

    Method

    Describe ligand-induced d-level splitting and a d–d transition.

    Reason

    A d electron absorbs a visible-light photon matching the energy gap, so remaining light appears coloured.

    Working

    Visible wavelength absorbed → observed colour.

Common misconception 2

Why Zn²⁺ Is Usually Colourless

Find and correct the mistake

Learner claim

A learner says Zn²⁺ is colourless because it has no d electrons. Correct the explanation.

Distinguish full from empty

Zn²⁺ configuration
Suitable d–d transition

View solution step by step

  1. Correct the configuration

    Method

    State 3d¹⁰, not d⁰.

    Reason

    The d subshell is full.

    Working

    Zn²⁺:3d¹⁰.

  2. Connect to colour

    Method

    State there is no suitable d–d transition.

    Reason

    Visible light is not significantly absorbed by this mechanism.

    Working

    Solution usually colourless.

Challenge 3

Colour Change after Ligand Exchange

Minimal support

Ligand transfer

A ligand exchange changes a complex’s colour. Explain the change using d-orbital splitting and wavelength.

Build the causal chain

New ligand changes
Therefore absorbed

Hints

Hint 1: ligand effect
Different ligands create different splitting magnitudes.
Hint 2: energy-light link
Photon energy determines wavelength.
View solution step by step

  1. Change the gap

    Method

    State the new ligand changes d-orbital splitting.

    Reason

    Ligand identity affects the metal–ligand field.

    Working

    Different ligand → different Δ E.

  2. Change absorbed light

    Method

    State a different wavelength is absorbed.

    Reason

    The transition must match the new energy gap, changing observed colour.

    Working

    Different absorption → different observed colour.

Mind Stretchers

Mind stretcher 1Extension

Suggest why increasing the oxidation state of a metal ion might change the colour of its complexes.

Show Hint

A higher oxidation state can alter metal–ligand attraction and therefore the separation of the split d levels.

Show Answer

Mark scheme:

  • Higher oxidation state increases attraction between the metal ion and ligands.
  • This can increase the splitting of the d orbitals, changing which wavelength is absorbed.

Mind stretcher 2: Separating colour intensity from colour identityExtension

Question. Diluting a coloured complex makes its solution paler without changing the complex ion. Ligand exchange changes its hue. Explain why only the second observation is evidence that the d-orbital energy gap changed.

Show Hint

Ask whether the identity of each complex changed or only the number of absorbing particles per unit volume.

Show Answer

Dilution reduces the number of absorbing complex ions per unit path length, so less light is absorbed and the same hue appears paler. The identity and octahedral splitting of each complex ion are unchanged. Ligand exchange changes the environment around the metal ion, so the splitting and absorbed wavelength can change; a different observed hue is therefore evidence for a changed energy gap.