Colour of complexes
Cambridge A-Level Chemistry (9701) · Unit 28: Chemistry of transition elements · 8 flashcards
Colour of complexes is topic 28.3 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 28 — Chemistry of transition elements , alongside Stereoisomerism in transition element complexes and Stability constants, Kstab. In one line: Degenerate d orbitals are d orbitals that possess the same energy level. In an isolated atom or ion, the five d orbitals are degenerate. This degeneracy is removed when ligands are introduced.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 8 flashcards — 2 definitions and 6 key concepts — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The term 'degenerate d orbitals'
Degenerate d orbitals are d orbitals that possess the same energy level. In an isolated atom or ion, the five d orbitals are degenerate. This degeneracy is removed when ligands are introduced.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Define what degenerate and non-degenerate mean for d orbitals, and use these terms.
- Describe how degenerate d orbitals split into two non-degenerate sets at different energies, and use ΔE, in: (a) octahedral complexes, with two orbitals higher and three lower; (b) tetrahedral complexes, with three orbitals higher and two lower.
- Explain why transition element compounds are coloured: light of a particular frequency is absorbed when an electron is promoted from a lower to a higher non-degenerate d orbital.
- Describe in qualitative terms how different ligands alter ΔE, which changes the frequency of light that is absorbed and therefore the complementary colour we see.
- Use the complexes that copper(II) and cobalt(II) ions form with water, ammonia, hydroxide ions and chloride ions as examples of ligand exchange changing the colour seen.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Colour of complexes
- › Explain color as: d-orbitals split, light is absorbed for electron promotion/excitation, and the complementary color is seen.
- › Full explanations must state that d-d transitions are impossible if d-orbitals are empty or completely full.
Define the term 'degenerate d orbitals'.
Degenerate d orbitals are d orbitals that possess the same energy level. In an isolated atom or ion, the five d orbitals are degenerate. This degeneracy is removed when ligands are introduced.
Describe how the d orbitals split in an octahedral complex.
In an octahedral complex, the five degenerate d orbitals split into two sets. Three d orbitals (dxy, dxz, dyz) are lower in energy, and two d orbitals (dz2, dx2-y2) are higher in energy. This energy difference is denoted as ΔE.
Describe how the d orbitals split in a tetrahedral complex.
In a tetrahedral complex, the five degenerate d orbitals split into two sets. Three d orbitals (dxy, dxz, dyz) are higher in energy, and two d orbitals (dz², dx²−y²) are lower in energy. This is the inverse of the octahedral splitting pattern, and ΔE is smaller.
Explain why transition metal compounds are coloured.
Transition metal compounds are coloured because electrons absorb specific frequencies of visible light to get promoted from a lower energy d orbital to a higher energy d orbital (d-d transition). The observed colour is the complementary colour to the light absorbed.
How does the identity of a ligand affect the magnitude of ΔE?
Different ligands cause different degrees of d-orbital splitting, and thus different values of ΔE. Strong-field ligands cause a large splitting (large ΔE), and weak-field ligands cause a small splitting (small ΔE).
Explain how ligand exchange can affect the observed colour of a complex.
Ligand exchange alters the magnitude of ΔE. This, in turn, changes the frequency of light absorbed and the complementary colour observed.
What happens to the frequency of light absorbed as ΔE increases?
As ΔE increases, the frequency of light absorbed also increases (E = hf). This means the compound will absorb light towards the blue/violet end of the spectrum and appear yellow/orange, as opposed to absorbing red/orange and appearing blue/green.
Give an example of a ligand exchange reaction using copper(II) ions and ammonia molecules.
The reaction between [Cu(H₂O)₆]²⁺ (pale blue) and ammonia (NH₃) initially forms a pale blue precipitate of copper(II) hydroxide. With excess ammonia, the precipitate dissolves, and [Cu(NH₃)₄(H₂O)₂]²⁺ (deep blue) is formed.
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Colour of complexes sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Colour of complexes deck
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