28.3 Colour of Complexes

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Define and use the terms degenerate and non-degenerate d orbitals.
  2. Describe how degenerate d orbitals split into two non-degenerate sets in (a) octahedral and (b) tetrahedral complexes, and explain the role of ΔE.
  3. Explain why transition elements form coloured compounds in terms of the frequency of light absorbed when an electron is promoted between non-degenerate d orbitals.
  4. Describe, in qualitative terms, how different ligands affect ΔE, the frequency of light absorbed, and therefore the complementary colour observed.
  5. Use complexes of copper(II) and cobalt(II) ions with water, ammonia, hydroxide, and chloride as examples of how ligand exchange changes the colour observed.

Section 1: Degenerate and Non-Degenerate d Orbitals

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What are d orbitals?

Every transition metal atom or ion has five d orbitals. Think of these as five "shelves" where electrons can sit. Each orbital has a specific shape and direction in 3D space.

The five d orbitals are named:

  • dxy, dxz, dyz — these three have lobes (bulging regions) that point between the x, y, and z axes.
  • dx²−y² and dz² — these two have lobes that point along the x, y, and z axes.

Degenerate d Orbitals

When a transition metal ion is isolated (on its own, not bonded to anything), all five d orbitals have exactly the same energy level. Orbitals that are equal in energy are called degenerate orbitals.

Degenerate orbitals = orbitals that are all at the same energy level.

Imagine five shelves all at the exact same height — electrons can sit on any of them equally.

Non-Degenerate d Orbitals

When the transition metal ion forms a complex (i.e., ligands attach to it via dative covalent bonds), the five d orbitals are no longer all equal in energy. They split into two groups at different energy levels. These are called non-degenerate orbitals.

Non-degenerate orbitals = orbitals that are at different energy levels (not equal).

The energy gap between the two groups is labelled ΔE (pronounced "delta E"). ΔE is the key to understanding colour.

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