Ligand exchange and redox
| English | Chinese | Pinyin |
|---|---|---|
| ligand exchange | 配体交换 | pèi tǐ jiāo huàn |
| redox titration | 氧化还原滴定 | yǎng huà huán yuán dī dìng |
| feasible | 可行 | kě xíng |
| thiosulfate | 硫代硫酸盐 | liú dài liú suān yán |
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:
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.
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:
Chloride ligands replace water, giving the yellow tetrachlorocuprate(II) ion.
Match each colour change.
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:
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:
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), $\text{Cu}^{2+}/\text{I}^-$