What you'll learn
The transition elements — the d-block metals from titanium to copper in the first row — have distinctive properties that make them some of the most useful substances in chemistry and industry. They form coloured compounds, show variable oxidation states, form complex ions, and are excellent catalysts. This guide explains what makes an element a transition element, their electronic configurations, why they have variable oxidation states, how complex ions form with ligands and what shapes they adopt, why their compounds are coloured (d-orbital splitting), ligand exchange reactions and stability constants, and their roles as catalysts — from iron in ammonia production at Point Lisas to the platinum-group metals in catalytic converters.
CAPE questions often ask you to write configurations of transition metal ions, explain colour, describe the formation and shapes of complexes, write ligand exchange equations with observations, and explain catalytic behaviour.
Key terms and definitions
Transition element — A d-block element that forms at least one stable ion with a partially filled d sub-level.
Ligand — A molecule or ion with a lone pair of electrons that forms a dative (coordinate) bond with a central metal ion.
Complex ion — A central metal ion surrounded by ligands bonded by dative bonds.
Coordination number — The number of dative bonds from ligands to the central metal ion.
Monodentate ligand — A ligand that forms one dative bond (e.g. H₂O, NH₃, Cl⁻, CN⁻).
Bidentate ligand — A ligand that forms two dative bonds (e.g. ethane-1,2-diamine, ethanedioate).
Polydentate (multidentate) ligand — A ligand forming several dative bonds (e.g. EDTA⁴⁻, which forms six).
Ligand exchange (substitution) — The replacement of one ligand by another in a complex.
Stability constant, K_stab — The equilibrium constant for the formation of a complex ion from the aqueous metal ion; larger values mean more stable complexes.
d-orbital splitting — The separation of d orbitals into two energy levels when ligands approach a metal ion.
Core concepts
Electronic configurations
The first-row transition elements fill the 3d sub-level:
- Ti: [Ar] 3d² 4s²
- V: [Ar] 3d³ 4s²
- Cr: [Ar] 3d⁵ 4s¹
- Mn: [Ar] 3d⁵ 4s²
- Fe: [Ar] 3d⁶ 4s²
- Co: [Ar] 3d⁷ 4s²
- Ni: [Ar] 3d⁸ 4s²
- Cu: [Ar] 3d¹⁰ 4s¹
When ions form, 4s electrons are lost before 3d: Fe²⁺ is [Ar] 3d⁶; Cu²⁺ is [Ar] 3d⁹.
Scandium and zinc are d-block elements but are not transition elements by this definition: Sc³⁺ is [Ar] (empty d) and Zn²⁺ is [Ar] 3d¹⁰ (full d), so neither forms an ion with a partially filled d sub-level. Their compounds are typically white or colourless.
General properties
Compared with s-block metals, transition metals have:
- higher densities and higher melting points (strong metallic bonding involving d electrons);
- similar atomic radii across the series (increasing nuclear charge is offset by added shielding from d electrons);
- relatively small changes in first ionisation energy across the series.
Variable oxidation states
Because the 4s and 3d electrons have similar energies, different numbers can be lost or used in bonding, giving several oxidation states. Manganese ranges from +2 (Mn²⁺) to +7 (MnO₄⁻); chromium shows +3 (Cr³⁺) and +6 (Cr₂O₇²⁻, CrO₄²⁻); iron shows +2 and +3. Higher oxidation states usually occur in oxoanions or compounds with highly electronegative elements (oxygen, fluorine).
Complex ions and shapes
Ligands donate lone pairs to empty orbitals on the metal ion.
| Coordination number | Shape | Example |
|---|---|---|
| 6 | octahedral (90°) | [Fe(H₂O)₆]²⁺, [Cu(H₂O)₆]²⁺ |
| 4 | tetrahedral (109.5°) | [CuCl₄]²⁻, [CoCl₄]²⁻ |
| 4 | square planar (90°) | [Pt(NH₃)₂Cl₂] (cisplatin) |
| 2 | linear (180°) | [Ag(NH₃)₂]⁺ (Tollens' reagent) |
Small ligands like water and ammonia usually give six-coordinate octahedral complexes; larger ligands such as Cl⁻ often give four-coordinate tetrahedral complexes.
Cisplatin, a square-planar platinum complex, is an anticancer drug that binds to DNA in cancer cells.
Why transition metal compounds are coloured
In an isolated ion, the five d orbitals have the same energy. When ligands surround the ion, the d orbitals split into two groups with slightly different energies. Electrons can absorb light energy and jump from the lower to the higher set: ΔE = hf. The frequencies absorbed are removed from white light, and the complementary colour is seen (transmitted or reflected).
Colour therefore requires partially filled d orbitals — ions with d⁰ (Sc³⁺, Ti⁴⁺) or d¹⁰ (Zn²⁺, Cu⁺) configurations are colourless.
The colour depends on:
- the metal and its oxidation state;
- the ligands (different ligands split d orbitals by different amounts);
- the coordination number and shape.
Colorimetry uses the absorbance of coloured solutions to measure concentration, since absorbance is proportional to concentration (Beer–Lambert law).
Ligand exchange
Copper(II):
- [Cu(H₂O)₆]²⁺ (pale blue) + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ (deep blue) + 4H₂O With a little ammonia, a pale blue precipitate of Cu(OH)₂ forms first; excess ammonia dissolves it to give the deep blue solution.
- [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ (yellow-green) + 6H₂O, with concentrated HCl. The coordination number changes from 6 to 4 because Cl⁻ is larger than H₂O.
Cobalt(II):
- [Co(H₂O)₆]²⁺ (pink) + 4Cl⁻ ⇌ [CoCl₄]²⁻ (blue) + 6H₂O
Stability: ligand exchange occurs when the new complex is more stable (larger K_stab). Polydentate ligands such as EDTA form very stable complexes because replacing six water molecules with one EDTA ion increases the number of particles in solution, which is entropically favourable — this is the chelate effect. EDTA is used to treat heavy-metal poisoning and to remove metal ions in water softening and food preservation.
Transition metals as catalysts
Transition metals and their compounds are effective catalysts because they:
- have variable oxidation states, allowing them to accept and donate electrons in intermediate steps (homogeneous catalysis);
- can adsorb reactant molecules onto their surfaces using d orbitals, weakening bonds and bringing molecules together (heterogeneous catalysis).
Examples:
- Iron in the Haber process (N₂ + 3H₂ ⇌ 2NH₃) — central to Trinidad's ammonia industry.
- Vanadium(V) oxide in the Contact process: V₂O₅ oxidises SO₂ to SO₃, being reduced to V₂O₄, then is reoxidised by oxygen.
- Nickel in the hydrogenation of vegetable oils to make margarine.
- Platinum, palladium and rhodium in catalytic converters.
- Fe²⁺/Fe³⁺ catalysing the reaction between peroxodisulfate and iodide ions.
Iron and haemoglobin
Haemoglobin contains Fe²⁺ in a complex that binds oxygen as a ligand, carrying it round the body. Carbon monoxide is toxic because it binds to the iron more strongly than oxygen does, forming a more stable complex and preventing oxygen transport.
Worked examples
Example 1: Configurations
Write the electronic configurations of Mn²⁺ and Cr³⁺.
Mn: [Ar] 3d⁵ 4s² → Mn²⁺: [Ar] 3d⁵. Cr: [Ar] 3d⁵ 4s¹ → Cr³⁺: [Ar] 3d³.
Example 2: Explaining colour
Explain why [Cu(H₂O)₆]²⁺ is coloured but [Zn(H₂O)₆]²⁺ is colourless. (3 marks)
Cu²⁺ has a 3d⁹ configuration with a partially filled d sub-level (1). The water ligands split the d orbitals, so electrons can absorb visible light and move to the higher energy level, and the complementary colour (blue) is seen (1). Zn²⁺ is 3d¹⁰, a full d sub-level, so no d–d transitions are possible and no visible light is absorbed (1).
Example 3: Ligand exchange
Describe what is seen when concentrated hydrochloric acid is added to aqueous copper(II) sulfate, and write an equation.
The pale blue solution turns yellow-green. [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O. The complex changes from octahedral to tetrahedral.
Example 4: Catalysis
Explain how V₂O₅ catalyses the Contact process.
V₂O₅ oxidises SO₂ to SO₃ and is itself reduced: SO₂ + V₂O₅ → SO₃ + V₂O₄. Oxygen then reoxidises the catalyst: 2V₂O₄ + O₂ → 2V₂O₅. The catalyst is regenerated, using its ability to change oxidation state.
Example 5: Oxidation states in complexes
Determine the oxidation state of the metal in (a) [Fe(CN)₆]³⁻, (b) [Cu(NH₃)₄(H₂O)₂]²⁺, (c) [CoCl₄]²⁻, (d) MnO₄⁻.
(a) CN⁻ has charge −1: Fe + 6(−1) = −3, so Fe = +3. (b) NH₃ and H₂O are neutral: Cu = +2. (c) Cl⁻ has charge −1: Co + 4(−1) = −2, so Co = +2. (d) O = −2: Mn + 4(−2) = −1, so Mn = +7.
Example 6: Using colorimetry with a complex
Describe how colorimetry can be used to find the concentration of copper(II) ions in a solution.
- Add excess aqueous ammonia to standard copper(II) solutions of known concentration to form the deep blue complex, which absorbs strongly.
- Select a filter of the complementary colour (orange/red) or set the wavelength to λmax.
- Measure the absorbance of each standard and plot a calibration curve of absorbance against concentration.
- Treat the unknown solution in the same way, measure its absorbance and read the concentration from the graph.
Example 7: The chelate effect
Explain why [Cu(EDTA)]²⁻ forms readily when EDTA⁴⁻ is added to [Cu(H₂O)₆]²⁺.
[Cu(H₂O)₆]²⁺ + EDTA⁴⁻ → [Cu(EDTA)]²⁻ + 6H₂O
One EDTA ion replaces six water molecules, so the number of particles in solution increases from two to seven. This large increase in entropy makes the reaction very favourable, and the resulting complex is very stable (it has a large stability constant). The bonds formed are similar in strength to those broken, so the enthalpy change is small; entropy drives the reaction.
Example 8: Distinguishing iron(II) and iron(III)
Describe two tests to distinguish aqueous Fe²⁺ from Fe³⁺.
- Sodium hydroxide: Fe²⁺ gives a green precipitate of Fe(OH)₂, which slowly turns brown in air; Fe³⁺ gives a red-brown precipitate of Fe(OH)₃ immediately.
- Potassium thiocyanate: Fe³⁺ gives a deep blood-red colour; Fe²⁺ gives little or no colour.
Properties of first-row transition metals at a glance
| Property | Reason |
|---|---|
| High melting points and densities | strong metallic bonding involving 3d and 4s electrons |
| Variable oxidation states | 3d and 4s electrons similar in energy |
| Coloured compounds | partially filled d orbitals split by ligands; d–d transitions absorb visible light |
| Complex ion formation | small, highly charged ions with empty orbitals that accept lone pairs |
| Catalytic activity | variable oxidation states and ability to adsorb reactants on surfaces |
Common mistakes and how to avoid them
- Removing 3d electrons before 4s when forming ions.
- Calling zinc a transition element.
- Saying the colour seen is the colour absorbed — it is the complementary colour.
- Forgetting that coordination number can change in ligand exchange (H₂O → Cl⁻).
- Confusing the two catalytic mechanisms — variable oxidation states (homogeneous) and adsorption (heterogeneous).
Self-test questions
- Write the electronic configuration of V³⁺. Answer: V is [Ar] 3d³ 4s²; V³⁺ is [Ar] 3d².
- Why is Ti⁴⁺ colourless? Answer: Ti⁴⁺ is [Ar] 3d⁰; with no d electrons, no d–d transitions can occur, so no visible light is absorbed.
- What is the coordination number and shape of [Ag(NH₃)₂]⁺? Answer: Coordination number 2; linear.
- Give one example of a transition metal acting as a heterogeneous catalyst and one acting as a homogeneous catalyst. Answer: Heterogeneous: iron in the Haber process. Homogeneous: Fe²⁺/Fe³⁺ ions catalysing the reaction between peroxodisulfate and iodide ions.
How this links to your Internal Assessment
Transition metal chemistry lends itself to visually clear SBAs: preparing complexes and observing ligand exchange colour changes, determining the formula of a complex by colorimetry (varying the ratio of metal ion to ligand and measuring absorbance), or using colorimetry to find the concentration of copper(II) ions in a solution. Good reports calibrate the colorimeter with standard solutions, plot a calibration curve, and evaluate errors such as cuvette cleanliness and wavelength choice.
Exam technique for Transition elements and complexes
- Define a transition element precisely, mentioning a stable ion with a partially filled d sub-level.
- When explaining colour, include splitting of d orbitals, absorption of light, electron promotion and complementary colour.
- For ligand exchange, give colours before and after, equations and any change in shape or coordination number.
- For catalysis, name the catalyst and explain whether it works by variable oxidation states or adsorption.
Quick revision summary
- Transition elements form stable ions with partially filled d sub-levels; Sc and Zn are excluded.
- Cr = [Ar] 3d⁵ 4s¹, Cu = [Ar] 3d¹⁰ 4s¹; 4s electrons are lost first.
- Variable oxidation states arise because 4s and 3d energies are similar.
- Complexes: ligands donate lone pairs; octahedral (6), tetrahedral (4), square planar (4), linear (2).
- Colour from d-orbital splitting and absorption of visible light; complementary colour seen.
- Ligand exchange: Cu²⁺ with NH₃ (deep blue) and Cl⁻ (yellow-green); Co²⁺ pink ⇌ blue; chelate effect and EDTA.
- Catalysts: Fe (Haber), V₂O₅ (Contact), Ni (hydrogenation), Pt/Pd/Rh (catalytic converters).