1. Principles of Aqueous Transition Metal Complex Ions
In aqueous solution, transition metal salts dissolve to form octahedral hexaaqua complex ions [M(H2O)6]^n+. Because metal cations polarise the O-H bonds of coordinated water molecules, these hexaaqua ions act as weak acids (Brønsted-Lowry acid-base hydrolysis). Adding basic reagents removes protons stepwise, forming insoluble neutral metal hydroxide complexes:
2. Diagnostic Qualitative Test Protocol
The OxfordAQA Section 6.1 practical protocol requires testing aqueous metal ion solutions systematically in clean test tubes:
- Place 10 drops (approx. 1 cm3) of the unknown metal ion solution into three separate clean test tubes labeled 1, 2, and 3.
- Tube 1 (Sodium Hydroxide): Add dilute 0.4 mol dm^-3 NaOH dropwise with gentle shaking. Record initial precipitate colour. Then add excess NaOH (approx. 2 to 3 cm3). Observe whether precipitate dissolves or remains insoluble.
- Tube 2 (Ammonia): Add dilute 2.0 mol dm^-3 NH3 dropwise with gentle shaking. Record initial precipitate colour. Then add excess ammonia (approx. 2 to 3 cm3). Observe whether precipitate dissolves (ligand substitution) to give a coloured solution.
- Tube 3 (Sodium Carbonate): Add 0.5 mol dm^-3 Na2CO3 dropwise. Observe whether a precipitate forms, note its colour, and carefully check for effervescence (fizzing of CO2 gas).
- Confirmatory Test (Conc. HCl): In a fume cupboard, add concentrated HCl dropwise to the metal ion solution. Observe ligand substitution forming tetrahedral chloro complexes (e.g. [CuCl4]^2- yellow-green).
3. Precipitates, Excess Reactions, and Characteristic Colours
| Metal Cation | Hexaaqua Ion | Dropwise NaOH or NH3 | Excess NaOH | Excess NH3 |
|---|---|---|---|---|
| Cu2+ | [Cu(H2O)6]^2+ (blue solution) |
Cu(OH)2(H2O)4(s) Blue precipitate |
Insoluble (remains blue ppt) |
Dissolves to form deep blue solution [Cu(NH3)4(H2O)2]^2+ |
| Fe2+ | [Fe(H2O)6]^2+ (pale green solution) |
Fe(OH)2(H2O)4(s) Green precipitate |
Insoluble (surface darkens to brown in air) |
Insoluble (surface darkens to brown) |
| Fe3+ | [Fe(H2O)6]^3+ (yellow-brown solution) |
Fe(OH)3(H2O)3(s) Rust-brown precipitate |
Insoluble (remains brown ppt) |
Insoluble (remains brown ppt) |
| Al3+ | [Al(H2O)6]^3+ (colourless solution) |
Al(OH)3(H2O)3(s) White precipitate |
Dissolves to form colourless solution [Al(OH)4]^- (amphoteric) |
Insoluble (remains white ppt) |
| Cr3+ | [Cr(H2O)6]^3+ (violet-green solution) |
Cr(OH)3(H2O)3(s) Grey-green precipitate |
Dissolves to form dark green solution [Cr(OH)6]^3- (amphoteric) |
Insoluble (or slightly soluble in very conc NH3 to purple) |
| Co2+ | [Co(H2O)6]^2+ (pink solution) |
Co(OH)2(H2O)4(s) Blue-green precipitate |
Insoluble (turns pink on standing) |
Dissolves to form straw-yellow solution [Co(NH3)6]^2+ (oxidises to brown) |
4. Deprotonation Reactions & Amphoteric Hydroxides
Deprotonation (Hydrolysis) by Base
Both OH^- and aqueous NH3 behave as Brønsted-Lowry bases, pulling protons off the coordinated water molecules:
Because the resulting complex carries no electrical charge, it cannot be effectively hydrated by surrounding water molecules, precipitating out as a solid.
Amphoteric Hydroxide Behavior
Aluminium(III) and Chromium(III) hydroxides are amphoteric: they react with both acids and excess strong bases:
- Al(OH)3(H2O)3 + OH^-:
Al(OH)3(H2O)3(s) + OH^-(aq) → [Al(OH)4]^-(aq) + 3H2O(l)(dissolves to a colourless solution). - Cr(OH)3(H2O)3 + 3OH^-:
Cr(OH)3(H2O)3(s) + 3OH^-(aq) → [Cr(OH)6]^3-(aq) + 3H2O(l)(dissolves to a dark green solution).
Importantly, ammonia (a weak base) does NOT provide a high enough concentration of hydroxide ions to dissolve aluminium hydroxide or chromium hydroxide precipitates.
5. Ligand Substitution Reactions
Excess Ammonia with Copper(II)
Unlike aluminium or iron, copper(II) hydroxide dissolves in excess concentrated ammonia because NH3 acts as a neutral Lewis base ligand, replacing water molecules via ligand substitution:
The resulting complex ion has an elongated octahedral geometry and a vivid deep blue colour. This reaction provides a definitive qualitative identification of Cu2+ ions.
Excess Concentrated Hydrochloric Acid (Chloro Complexes)
Adding concentrated HCl (a source of Cl^- ligands) causes complete ligand displacement accompanied by a change in coordination number from 6 to 4 (octahedral to tetrahedral), driven by the larger size of chloride ligands:
6. Diagnostic Reactions with Sodium Carbonate (2+ vs 3+ Ions)
Sodium carbonate solution (Na2CO3) distinguishes 2+ from 3+ metal cations reliably based on the acid-base properties of the metal ions:
2+ Cations (Cu2+, Fe2+, Co2+)
Lower charge density means hexaaqua 2+ ions are not sufficiently acidic to hydrolyse carbonate ions. Instead, a simple precipitation reaction occurs, producing insoluble metal carbonates with no effervescence:
[Cu(H2O)6]^2+(aq) + CO3^2-(aq) → CuCO3(s) + 6H2O(l)
CuCO3 is a blue-green precipitate; FeCO3 is a green precipitate.
3+ Cations (Fe3+, Al3+, Cr3+)
Higher charge density strongly polarises coordinated water ligands, making 3+ hexaaqua ions significantly more acidic. They react with carbonate ions in an acid-base neutralization, releasing carbon dioxide gas (effervescence) and precipitating the metal hydroxide:
2[Fe(H2O)6]^3+(aq) + 3CO3^2-(aq) → 2Fe(OH)3(H2O)3(s) + 3CO2(g) + 3H2O(l)
Observe effervescence and a rust-brown precipitate for Fe3+, or white precipitate for Al3+.
7. Practice Questions
Question 1: An unknown solution X forms a green precipitate when treated dropwise with dilute sodium hydroxide. On standing in air, the top of the precipitate turns brown. Adding sodium carbonate produces a green precipitate with no gas evolution. What is the identity of the metal cation?
Show Answer & Explanation
Correct Answer: B
Iron(II) forms green Fe(OH)2(H2O)4(s), which oxidises in contact with atmospheric oxygen to brown Fe(OH)3(H2O)3(s). As a 2+ ion, it reacts with sodium carbonate without effervescence to form green FeCO3.
Question 2: Which observation distinguishes aqueous aluminium(III) from aqueous iron(III) when tested with sodium hydroxide and sodium carbonate?
Show Answer & Explanation
Correct Answer: B
Both 3+ ions produce effervescence (CO2) with sodium carbonate. However, aluminium hydroxide is amphoteric and dissolves in excess NaOH to give colourless [Al(OH)4]^-(aq), whereas iron(III) hydroxide is strictly basic and remains insoluble as a rust-brown precipitate.