Why are the ions of transition metals in solution often coloured?

They absorb part of the visible spectrum when an electron moves between d energy levels, so the light we see is missing those wavelengths.

What effect do the ligands have on the d-orbitals of the central metal ion?

They split them into two groups of slightly different energy.

What must be true of the d sub-shell for a complex to be coloured?

It must be partly filled, so that there is an electron able to move to a higher level and a vacancy to move into.

Which two common d-block ions are colourless, and why?

Sc³⁺ is colourless because it has no d electrons and Zn²⁺ is colourless because its 3d sub-shell is completely full.

Describe what happens when visible light passes through a solution of a coloured complex.

A photon whose energy exactly matches the gap between the split d levels is absorbed and an electron is promoted to the higher level.

OCR A-Level Chemistry B H433

Development of Practical Skills in Chemistry

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Elements of Life

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Elements From the Sea

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Oceans

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Developing Metals

Topic overview Manganate(VII) titrations and their calculations Thiosulfate titrations, recording results and good technique Shapes of complexes and coordination number Balancing redox equations involving acid-base reactions Simple electrochemical cells Action of an electrochemical cell Standard electrode potentials and calculating E°cell Rusting and how it is prevented Transition metals: definition, oxidation states and colours d-block electron configurations of the atoms and their ions Why transition metals show variable oxidation states Ligands and complex ions Coordinate bonding and what makes a ligand Bidentate and multidentate ligands, and finding a complex's formula Reactions of Fe2+, Fe3+ and Cu2+ with hydroxide and ammonia Catalytic activity of transition metals Origin of colour in transition metal complexes Colorimetry

Colour by Design

Topic overview How dyes attach to fibres Structure of a dye molecule Fats and oils Arenes and delocalisation Representations of benzene and enthalpy of hydrogenation Tests for the common functional groups Identifying a polyfunctional molecule from a set of test results Halogenation and nitration of benzene and the electrophiles involved The nitration mechanism, sulfonation and the Friedel-Crafts reactions Diazotisation and why it is done cold Coupling reactions, azo dyes and the route from benzene Oxidising an aldehyde and the three carbonyl tests Cyanohydrin formation and its use in synthesis Using the Data Sheet reactions to plan a route Choosing between routes on yield and atom economy Nucleophilic addition mechanism Classifying organic reactions and unfamiliar mechanisms Origins of colour in organic molecules How gas-liquid chromatography separates a mixture Retention time, peak area and the limits of the technique

Chemical Literacy

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Mathematical Skills in Chemistry

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Developing Metals

Origin of colour in transition metal complexes

4 things to know
Components 01, 02 and 03

Overview

Recall questions on d orbital splitting, why some ions are colourless, and how ligand, oxidation state and coordination number change the colour.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Which two d-block ions are colourless and why, why do we see a colour rather than darkness, and how is the energy gap related to the frequency absorbed?

Process

What does a larger energy gap do to the wavelength absorbed, why does ammonia deepen a copper(II) solution, and how does oxidation state change the colour of aqueous iron?

Key term

How can coordination number change a copper complex's colour, why is a dilute solution paler, and why do transitions between the split d levels absorb visible rather than ultraviolet light?

Numbers

Combine ΔE = hν and c = νλ to link the energy gap to the wavelength absorbed.

Questions from this subtopic

Each one with the model answer underneath.

1
Why are the ions of transition metals in solution often coloured?
Model answer
They absorb part of the visible spectrum when an electron moves between d energy levels, so the light we see is missing those wavelengths.
2
What effect do the ligands have on the d-orbitals of the central metal ion?
Model answer
They split them into two groups of slightly different energy.
3
What must be true of the d sub-shell for a complex to be coloured?
Model answer
It must be partly filled, so that there is an electron able to move to a higher level and a vacancy to move into.
4
Which two common d-block ions are colourless, and why?
Model answer
Sc³⁺ is colourless because it has no d electrons and Zn²⁺ is colourless because its 3d sub-shell is completely full.
5
Describe what happens when visible light passes through a solution of a coloured complex.
Model answer
A photon whose energy exactly matches the gap between the split d levels is absorbed and an electron is promoted to the higher level.
6
Why do we see a colour rather than darkness?
Model answer
Only some wavelengths are absorbed, so the light that passes through is the remainder, and we see the complementary colour.

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