Lesson 4 of 6
Ligands, complexes and observations
Follow copper colours, QA evidence and haemoglobin exchange.
A-Level 9476 (2026-2027)
I can define ligands and complexes using required examples.Syllabus
Syllabus 9476, 13(i). Define ligands and complexes using required examples.
I can explain ligand exchange and associated colours.Syllabus
Syllabus 9476, 13(j). Explain ligand exchange and associated colours.
Ligands donate electron pairs to a metal centre
A complex can be positive, neutral or negative.
Make a guess. You are not marked.
Excess concentrated hydrochloric acid is added to blue aqueous copper(II) sulfate. What do you see?
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The solution turns yellow-green as [CuCl4]2- forms
Chloride ions replace the water ligands to form [CuCl4]2-, which absorbs light differently from [Cu(H2O)6]2+.
A ligand is an ion or molecule that donates a lone pair to a central metal atom or ion to form a coordinate bond. A complex contains that central metal bonded to its surrounding ligands. H2O and NH3 are neutral donors through O and N respectively; Cl- is a negatively charged donor. The metal accepts the pair: it does not supply both electrons in that coordinate bond.
| Complex | Ligands and coordination | Overall charge |
|---|---|---|
| [Cu(H2O)6]2+ | Six water donors; coordination number 6. | +2: all water ligands are neutral. |
| [Cu(NH3)4(H2O)2]2+ | Four ammonia and two water donors; coordination number 6. Often abbreviated [Cu(NH3)4]2+ in equations. | +2: ammonia and water are neutral. |
| [CuCl4]2- | Four chloride donors; coordination number 4. | +2 + 4(-1) = -2. |
Coordination number counts donor atoms directly bonded to the metal. For these one-donor ligands it equals the number of ligands. Determine oxidation state separately from overall complex charge: the negative chloride complex still contains copper in the +2 state.
Two ligand changes, the same copper oxidation state
Water gives the blue copper(II) aqua complex. Excess ammonia produces a deep-blue ammine complex; concentrated chloride produces a yellow chloro complex. Both retain copper(II).
Excess ammonia: [Cu(H2O)6]2+(aq) + 4NH3(aq) ⇌ [Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l). Ammonia replaces four coordinated water molecules; the solution becomes deep blue.
Concentrated chloride: [Cu(H2O)6]2+(aq) + 4Cl-(aq) ⇌ [CuCl4]2-(aq) + 6H2O(l). High chloride concentration favours the yellow complex. Dilution favours the blue aqua form. These are ligand exchanges, not redox reactions.
Check your understanding
What is the oxidation state of chromium in [Cr(OH)6]3-?
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+3, since x + 6(-1) = -3
Each OH- ligand is -1. The overall charge includes the ligands, so x - 6 = -3 and x = +3.
Separate precipitation, ligand exchange and oxidation
The sequence of additions explains the qualitative-analysis observation.
A few drops of aqueous ammonia supply OH- through NH3 + H2O ⇌ NH4+ + OH-. Copper(II) first forms blue Cu(OH)2(s): Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s). In excess ammonia, complex formation removes dissolved copper ions and the precipitate dissolves to the deep-blue ammine solution. NaOH does not provide that ammonia ligand, so the precipitate remains in excess NaOH.
| Ion | NaOH(aq), then excess | NH3(aq), then excess |
|---|---|---|
| Cr3+ | Grey-green Cr(OH)3 precipitate; dissolves in excess to dark-green [Cr(OH)6]3-. | Grey-green precipitate; insoluble in excess. |
| Cu2+ | Pale-blue Cu(OH)2 precipitate; insoluble in excess. | Blue precipitate; dissolves in excess to a deep-blue complex. |
| Fe2+ | Green Fe(OH)2 precipitate; insoluble in excess; turns brown in air. | Same precipitate and excess-reagent behaviour. |
| Fe3+ | Red-brown Fe(OH)3 precipitate; insoluble in excess. | Same precipitate and excess-reagent behaviour. |
| Mn2+ | Off-white Mn(OH)2 precipitate; insoluble in excess; rapidly turns brown in air. | Same precipitate and excess-reagent behaviour. |
Chromium(III) hydroxide dissolves in excess hydroxide by forming a soluble hydroxo complex: Cr(OH)3(s) + 3OH-(aq) ⇌ [Cr(OH)6]3-(aq). Chromium remains +3. In contrast, the browning of freshly formed iron(II) or manganese(II) hydroxide in air involves oxidation. Record the initial colour and subsequent change separately.
Watch out for this
A student says: "The solution changed colour, so the metal ion must have been oxidised or reduced." What is wrong with this?
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Cr(III) solution colours vary with the ligands present; the usual green reaction mixture is not proof that every pure hexaqua chromium(III) solution must be green. A colour change can come from redox, ligand exchange or precipitation. Use the reagent, formula and phase as well as the colour. Complex formation is also possible for non-transition species, including zinc and silver ions.
In haemoglobin, oxygen binds reversibly to an iron-containing centre. Carbon monoxide competes for the binding site and binds much more strongly, displacing oxygen and reducing oxygen transport. A simplified exchange is HbO2 + CO ⇌ HbCO + O2. This illustrates ligand competition; it is not a reaction in which CO burns with oxygen.
Check your understanding
A blue solution gives a blue precipitate with a little ammonia, then a deep-blue solution with excess. What happens in each stage?
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First NH3 acts as a base, making Cu(OH)2; in excess it acts as a ligand, making a soluble complex.
A little ammonia supplies OH- and precipitates Cu(OH)2. Excess ammonia replaces ligands to form the deep-blue copper(II) ammine complex. Copper stays +2 throughout.