Edexcel International A Level Chemistry
Topic 17: Transition Metals and their Chemistry
Why d-block metals are special, how their ions form coloured complexes with ligands, and how the same electrons that give variable oxidation numbers make them good catalysts.
The one idea
A transition metal ion has an incompletely filled 3d sub-shell. That one fact explains almost everything in this topic:
- the 4s and 3d electrons are close in energy, so there are several oxidation numbers;
- the ion has empty orbitals, so it accepts lone pairs from ligands and forms complex ions with fixed shapes;
- ligands split the d-orbitals, so electrons absorb visible light and the ions are coloured, and the colour changes whenever the oxidation number, ligand or coordination number changes;
- the metal can gain and lose electrons easily, so it is a good catalyst.
When you meet a new observation, ask the same three questions: what is the oxidation number, what are the ligands, and what is the coordination number? The answer to the colour, shape or reaction follows from them.
How to use these notes
- Read the pages in order the first time. Pages 1 to 7 build the ideas: electrons, ligands, shapes, colour. Pages 8 to 10 use E° values for vanadium and chromium. Pages 11 to 16 cover reactions of complex ions, from precipitates to haemoglobin. Page 17 is Core Practical 14. Pages 18 to 20 are catalysis.
- Each page opens with its one idea in a sentence.
- Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, and beyond the spec (interesting, but you will not be examined on it).
- Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
- Use the topic on a page the night before an exam. It has the observation tables and a mixed test.
Relative atomic masses are the Edexcel IAL periodic table values to one decimal place: H 1.0, C 12.0, O 16.0, K 39.1, Cr 52.0. Standard electrode potentials in worked examples are the values used in these notes; your exam gives its own data, so always use the values printed on the paper. Colours in the figures are schematic: check them against the colours you see in the laboratory.
Pages
- Transition metals and their electrons
A transition metal is defined by its ions, not its atoms. Work out the electron configuration of an ion first, then ask whether its 3d sub-shell is partly filled.
- Variable oxidation number
Transition metals form ions with several different charges because the 4s and 3d electrons are close in energy, so removing a few more or a few fewer costs similar energy.
- Ligands and complex ions
A complex ion is a central metal ion with ligands attached by dative bonds. Learn to count the bonds, work out the charge, and draw the ion.
- Shapes of complex ions
Six ligands give an octahedral ion, four large ligands give a tetrahedral ion, and a few four-coordinate ions are square planar. Cis-platin is the square planar example to know.
- Bidentate and hexadentate ligands
Some ligands have more than one atom that can donate a lone pair. They wrap round the metal ion and form two, or six, dative bonds. Count bonds, not ligands.
- Why complexes are coloured
Ligands split the five d-orbitals into two energy levels. A d electron absorbs visible light of energy equal to the gap and moves up. The light that is left is the colour you see.
- No colour, and changes of colour
An ion is colourless when no d electron can be promoted, which happens with empty and full d sub-shells. The colour changes when the oxidation number, the ligands or the coordination number changes the splitting.
- Vanadium: four oxidation states
Vanadium has four colourful oxidation states, +5 to +2. A reducing agent takes it down a step only if the cell potential for that step is positive, so the choice of reagent decides where it stops.
- Chromium: reduction and oxidation
Zinc in acid reduces orange dichromate(VI) through green Cr³⁺ to blue Cr²⁺. Going back up, hydrogen peroxide oxidises Cr³⁺ to chromate(VI), but only in alkaline conditions.
- Chromate(VI) and dichromate(VI)
Yellow chromate(VI) and orange dichromate(VI) are linked by an acid-base equilibrium. Alkali moves it to yellow and acid moves it to orange. Chromium is +6 in both, so it is not redox.
- Three reactions of aqua ions
Hydroxide ions and ammonia do three different things to a metal aqua ion. They remove H⁺ from water ligands (deprotonation), some hydroxides dissolve in excess alkali (amphoteric), and ammonia can replace ligands.
- Eight ions with sodium hydroxide
Every one of the eight ions gives a hydroxide precipitate with sodium hydroxide. Learn the colours, which precipitates dissolve in excess, and which darken in air.
- Eight ions with ammonia
A few drops of ammonia give the same precipitates as sodium hydroxide, because ammonia is a base. In excess, ammonia is a ligand and five of the precipitates dissolve.
- Ligand exchange and colour
Replacing water ligands by ammonia or by chloride ions changes the colour. Ammonia keeps six ligands round copper; chloride gives four, so the shape changes too.
- Stability and entropy
Replacing monodentate ligands with a bidentate or hexadentate ligand releases more particles into solution. That raises the entropy of the system, so the reaction is feasible and the new complex is more stable.
- Haemoglobin and carbon monoxide
Haemoglobin carries oxygen as a ligand on iron(II). Carbon monoxide replaces the oxygen by ligand exchange and binds much more strongly, so the blood can no longer carry oxygen.
- Core Practical 14: preparing a transition metal complex
Reduce dichromate(VI) with zinc and acid to blue chromium(II), then run it into sodium ethanoate to make the red complex chromium(II) ethanoate. Air must be kept out throughout.
- Two kinds of catalyst
Heterogeneous catalysts are in a different phase from the reactants and work on their surface. Homogeneous catalysts are in the same phase and work through an intermediate. Transition metals are good at both.
- V₂O₅ and catalytic converters
Vanadium(V) oxide speeds up the contact process by cycling between +5 and +4. In a catalytic converter, CO and NO stick to the surface of platinum and rhodium, react, and the products leave.
- Homogeneous catalysis in solution
Fe²⁺ speeds up the reaction of two negative ions by reacting with each in turn, with Fe³⁺ as the intermediate. Mn²⁺ is a product that catalyses its own formation, so the reaction starts slowly and then speeds up.
Also in this topic: Topic 17 on a page
Spec checklist
Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.
| Spec | You should be able to | Where |
|---|---|---|
| 17.1 | know that transition metals are d-block elements that form one or more stable ions with incompletely-filled d-orbitals | What a transition metal is |
| 17.2 | be able to deduce the electronic configurations of atoms and ions of the d-block elements of Period 4 (Sc-Zn) given their atomic number and charge (if any) | Configurations of Sc to Zn Configurations of ions |
| 17.3 | understand why transition metals show variable oxidation number | Close in energy Why the extra energy is affordable |
| 17.4 | know what is meant by the term 'ligand' | Ligands and dative bonds |
| 17.5 | understand that dative (coordinate) covalent bonding is involved in the formation of complex ions | Ligands and dative bonds |
| 17.6 | know that a complex ion is a central metal ion surrounded by ligands | Complex ions |
| 17.7 | know that aqueous solutions of transition metal ions are usually coloured | Coloured aqueous ions |
| 17.8 | understand that the colour of aqueous ions, and other complex ions, is a consequence of the splitting of the energy levels of the d-orbitals by ligands | Splitting the d-orbitals Which colour do you see? |
| 17.9 | understand why there is a lack of colour in some aqueous ions and other complex ions | When there is no colour |
| 17.10 | understand the meaning of the term 'coordination number' | Coordination number |
| 17.11(i) | understand that colour changes in transition metal ions may arise as a result of changes in: oxidation number of the ion | A change of oxidation number |
| 17.11(ii) | understand that colour changes in transition metal ions may arise as a result of changes in: ligand | A change of ligand |
| 17.11(iii) | understand that colour changes in transition metal ions may arise as a result of changes in: coordination number of the complex | A change of coordination number |
| 17.12 | understand that H₂O, OH⁻ and NH₃ act as monodentate ligands | Complex ions |
| 17.13 | understand why complexes with six-fold coordination have an octahedral shape, such as those formed by metal ions with H₂O, OH⁻ and NH₃ as ligands | Six ligands: octahedral |
| 17.14 | know that transition metal ions may form tetrahedral complexes with relatively large ions such as Cl⁻ | Four large ligands: tetrahedral |
| 17.15 | know that square planar complexes are also formed by transition metal ions and that cis-platin is an example of such a complex which is used in cancer treatment where it is supplied as a single isomer and not in a mixture with the trans form | Four ligands in one plane: square planar Cis-platin and its isomer |
| 17.16 | understand the terms 'bidentate' and 'hexadentate' in relation to ligands, and be able to identify examples such as NH₂CH₂CH₂NH₂ and EDTA⁴⁻ | Counting donor atoms 1,2-diaminoethane: a bidentate ligand EDTA: a hexadentate ligand |
| 17.17 | know that haemoglobin is an iron(II) complex containing a polydentate ligand and that ligand exchange occurs when an oxygen molecule bound to haemoglobin is replaced by a carbon monoxide molecule. The structure of the haem group will not be assessed. | Iron(II) in haemoglobin Carrying oxygen Carbon monoxide poisoning |
| 17.18 | know the colours of the oxidation states of vanadium (+5, +4, +3 and +2) in its compounds | Four colours |
| 17.19 | understand redox reactions for the interconversion of the oxidation states of vanadium (+5, +4, +3 and +2), in terms of the relevant E° values | The potentials Stopping at a chosen state |
| 17.20(i) | understand, in terms of the relevant E values, that the dichromate(VI) ion, Cr₂O₇²⁻: can be reduced to Cr³⁺ and Cr²⁺ ions using zinc in acidic conditions | Three oxidation states Reduction by zinc in acid The method |
| 17.20(ii) | understand, in terms of the relevant E values, that the dichromate(VI) ion, Cr₂O₇²⁻: can be produced by the oxidation of Cr³⁺ ions using hydrogen peroxide in alkaline conditions (followed by acidification) | Oxidation by hydrogen peroxide in alkali Why alkaline? |
| 17.21 | know that the dichromate(VI) ion, Cr₂O₇²⁻ can be converted into chromate(VI) ions as a result of the equilibrium Cr₂O₇²⁻ + H₂O ⇌ 2CrO₄²⁻ + 2H⁺ | The equilibrium The whole chromium story |
| 17.22 | be able to record observations and write suitable equations for the reactions of Cr³⁺(aq), Mn²⁺(aq), Fe²⁺(aq), Fe³⁺(aq), Co²⁺(aq), Ni²⁺(aq), Cu²⁺(aq) and Zn²⁺(aq) with aqueous sodium hydroxide and aqueous ammonia, including in excess | The observations The precipitates The two that dissolve The observations Ammonia as a base Ammonia as a ligand |
| 17.23 | be able to write ionic equations to show the meaning of amphoteric behaviour, deprotonation and ligand exchange in the reactions in 17.22 | Deprotonation Amphoteric behaviour Ligand exchange The precipitates The two that dissolve Ammonia as a base Ammonia as a ligand |
| 17.24(i) | understand that ligand exchange, and an accompanying colour change, occurs in the formation of: [Cu(NH₃)₄(H₂O)₂]²⁺ from [Cu(H₂O)₆]²⁺ via Cu(OH)₂(H₂O)₄ | Copper(II) with ammonia |
| 17.24(ii) | understand that ligand exchange, and an accompanying colour change, occurs in the formation of: [CuCl₄]²⁻ from [Cu(H₂O)₆]²⁺ | Copper(II) and cobalt(II) with chloride |
| 17.24(iii) | understand that ligand exchange, and an accompanying colour change, occurs in the formation of: [CoCl₄]²⁻ from [Co(H₂O)₆]²⁺ | Copper(II) and cobalt(II) with chloride |
| 17.25 | understand, in terms of the positive increase in ΔSsystem, that the substitution of a monodentate ligand by a bidentate or hexadentate ligand leads to a more stable complex ion | What "more stable" means Counting particles Why the enthalpy change is small |
| 17.26 | know that transition metals and their compounds can act as heterogeneous and homogeneous catalysts | Phase decides the kind Why transition metals |
| 17.27 | know that a heterogeneous catalyst is in a different phase from the reactants and that the reaction occurs at the surface of the catalyst | Phase decides the kind Catalytic converters |
| 17.28 | understand, in terms of oxidation number, how V₂O₅ acts as a catalyst in the contact process | The contact process |
| 17.29(i) | understand how a catalytic converter decreases carbon monoxide and nitrogen monoxide emissions from internal combustion engines by: adsorption of CO and NO molecules onto the surface of the catalyst, resulting in the weakening of bonds and chemical reaction | Catalytic converters |
| 17.29(ii) | understand how a catalytic converter decreases carbon monoxide and nitrogen monoxide emissions from internal combustion engines by: desorption of CO₂ and N₂ product molecules from the surface of the catalyst | Catalytic converters |
| 17.30 | know that a homogeneous catalyst is in the same phase as the reactants and appreciate that the catalysed reaction will proceed via an intermediate species | Phase decides the kind Fe²⁺ with iodide and peroxodisulfate |
| 17.31 | understand the role of Fe²⁺ ions in catalysing the reaction between I⁻ and S₂O₈²⁻ ions | Fe²⁺ with iodide and peroxodisulfate |
| 17.32 | know the role of Mn²⁺ ions in autocatalysing the reaction between MnO₄⁻ and C₂O₄²⁻ ions | Autocatalysis by Mn²⁺ |
| 17.33 | Core practical 14: The preparation of a transition metal complex. | Aim and chemistry The method Yield |