Why learn about coordination compounds? 🌿
Transition metals often team up with ions or neutral molecules to form coordination compounds. They show up everywhere—from chlorophyll and haemoglobin to industrial catalysts, textile dyes and medicines :contentReference[oaicite:0]{index=0}.
Werner’s big ideas (1898) 💡
- Two valences: primary (ionisable) and secondary (non-ionisable) :contentReference[oaicite:1]{index=1}.
- Primary valence is satisfied by negative ions (e.g., \(\text{Cl}^-\)).
- Secondary valence equals the coordination number and is fixed for each metal.
- Groups attached through secondary valence arrange in definite 3-D shapes called coordination polyhedra (octahedral, tetrahedral, square-planar) :contentReference[oaicite:2]{index=2}.
Werner’s classic experiment 🧪 (Cobalt(III) chloride + ammonia)
Silver-nitrate tests showed how many \(\text{Cl}^-\) were free to precipitate as \(\text{AgCl}\). The rest stayed bonded to cobalt inside the complex.
| Colour 🎨 | Formula | AgCl moles per mole of compound | Electrolyte type |
|---|---|---|---|
| Yellow | \([\text{Co(NH}_3)_6]^{3+} 3\text{Cl}^- \) | 3 | 1 : 3 |
| Purple | \([\text{CoCl(NH}_3)_5]^{2+} 2\text{Cl}^- \) | 2 | 1 : 2 |
| Green | \([\text{CoCl}_2\text{(NH}_3)_4]^+ \text{Cl}^- \) | 1 | 1 : 1 |
| Violet | \([\text{CoCl}_2\text{(NH}_3)_4]^+ \text{Cl}^- \) | 1 | 1 : 1 |
Same empirical formula, different properties = isomers! 🔄 :contentReference[oaicite:3]{index=3}
Primary vs secondary valence — quick practice ✅
Given the silver-nitrate data, Werner assigned secondary valences like this:
| (i) | \(\text{PdCl}_2·4\text{NH}_3\) | Secondary valence = 4 |
|---|---|---|
| (ii) | \(\text{NiCl}_2·6\text{H}_2\text{O}\) | Secondary valence = 6 |
| (iii) | \(\text{PtCl}_4·2\text{HCl}\) | Secondary valence = 6 |
| (iv) | \(\text{CoCl}_3·4\text{NH}_3\) | Secondary valence = 6 |
| (v) | \(\text{PtCl}_2·2\text{NH}_3\) | Secondary valence = 4 |
Notice how the \(\text{AgCl}\) test tells us the ionisable (primary) chloride count, while what stays inside the brackets counts toward the secondary valence :contentReference[oaicite:4]{index=4}.
Shapes you should know 📐
- Octahedral (6): \([\text{Co(NH}_3)_6]^{3+}\), \([\text{CoCl(NH}_3)_5]^{2+}\) etc.
- Tetrahedral (4): \([\text{Ni(CO)}_4]\)
- Square planar (4): \([\text{PtCl}_4]^{2-}\)
Shape depends on the fixed coordination number of the metal ion :contentReference[oaicite:5]{index=5}.
Double salt vs complex 🤔
- Double salts (e.g., \(\text{KCl·MgCl}_2·6\text{H}_2\text{O}\)) fully dissociate in water into simple ions.
- Complexes keep their complex ion intact in solution; only the counter-ions separate. Example: \(\text{K}_4[\text{Fe(CN)}_6] \rightarrow 4\text{K}^+ + [\text{Fe(CN)}_6]^{4-}\) (no release of \(\text{CN}^-\)) :contentReference[oaicite:6]{index=6}.
Meet Alfred Werner 👨🔬
Born in 1866, he became a professor at 29 and won the 1913 Nobel Prize for this very theory. Fun fact: he was first to observe optical activity in coordination compounds! 🎖️ :contentReference[oaicite:7]{index=7}
High-Yield NEET Pointers 🌟
- Primary vs secondary valence concept and how the silver-nitrate test reveals ionisable \(\text{Cl}^-\).
- Fixed coordination number leads to predictable geometries (octahedral, tetrahedral, square-planar).
- Difference between double salts and complexes based on dissociation behaviour in water.
- AgCl precipitation data as a practical method to count counter-ions in solution.
- Isomerism in coordination compounds (e.g., the two \(\text{CoCl}_3·4\text{NH}_3\) isomers).
🧠 Keep these highlights in mind & coordination chemistry will feel much friendlier!
