Ligand exchange and redox
Introduced| English | Meaning |
|---|---|
| ligand exchange/ˈlɪɡænd eksˈtʃeɪndʒ/ | A reaction in which one ligand in a complex is replaced by another. |
| redox titration/rɪˈdɒks taɪˈtreɪʃn/ | redox titration |
| feasible/ˈfiːzɪbl/ | Thermodynamically possible under the stated conditions; this does not establish that a reaction is fast. |
| thiosulfate/ˌθaɪəʊˈsʌlfeɪt/ | thiosulfate |
Swapping the surroundings
- In ligand exchange 配体交换, one ligand replaces another — often with a colour change.
- Copper(II) is the classic example.
- Transition ions also do useful redox titrations 氧化还原滴定.
Ligand exchange route
Follow one ligand replacing another around a metal ion.
Ligand exchange is when:
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Swapping a ligand for another changes the complex and usually its colour.
In ligand exchange, one ligand is replaced by another around the central metal ion.
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It can change the complex's colour.
Ligand exchange (copper II)
- $[\text{Cu}(\text{H}_2\text{O})_6]^{2+}$ is pale blue.
- Add ammonia → $[\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$, deep blue.
- Add concentrated HCl → $[\text{CuCl}_4]^{2-}$, yellow.
Ligand exchange changes the colour of copper(II)
Adding concentrated HCl to pale-blue [Cu(H₂O)₆]²⁺ gives:
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Chloride ligands replace water, giving the yellow tetrachlorocuprate(II) ion.
Match each colour change.
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Each item links the term to its correct meaning.
Redox titrations
- Use $E^{\ominus}$ values to predict whether a redox reaction is feasible 可行.
- Classic titrations: $\text{MnO}_4^-$ with $\text{Fe}^{2+}$ (purple → colourless), and $\text{Cu}^{2+}$ with $\text{I}^-$ (makes iodine, then titrated with thiosulfate 硫代硫酸盐).
In a MnO₄⁻/Fe²⁺ titration in acid, the colour change at the end point is:
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MnO₄⁻ (purple) is reduced to colourless Mn²⁺; the first excess drop gives a faint pink.
To predict whether a redox reaction of transition ions happens, you use:
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A positive E°cell (from the E° values) tells you the redox reaction is feasible.
Exchange can change colour
- Swapping water ligands for ammonia or chloride often changes the complex's colour.
- Ligand exchange can also change the coordination number (e.g. with larger Cl⁻).
You've got it
- ligand exchange swaps one ligand for another (colour change)
- copper(II): pale blue (water) → deep blue (ammonia) → yellow ($[\text{CuCl}_4]^{2-}$)
- use $E^{\ominus}$ for feasibility; classic titrations: $\text{MnO}_4^-/\text{Fe}^{2+}$ (purple → colourless), $\text{Cu}^{2+}/\text{I}^-$