Stability constants
Introduced| English | Meaning |
|---|---|
| stability constant/stəˈbɪlɪti ˈkɒnstənt/ | The equilibrium constant for forming a complex from its metal ion and ligands. |
| ligand exchange/ˈlɪɡænd eksˈtʃeɪndʒ/ | A reaction in which one ligand in a complex is replaced by another. |
| chelate effect/ˈtʃeleɪt ɪˈfekt/ | chelate effect |
| multidentate/ˌmʌltɪˈdenteɪt/ | multidentate |
| entropy/ˈentrəpi/ | A thermodynamic measure related to energy dispersal and the number of microscopic arrangements consistent with a system's macroscopic state. |
How stable is the complex?
- The stability constant 稳定常数 $K_{\text{stab}}$ measures how stable a complex is.
- It is the equilibrium constant for forming the complex in solution.
- It decides the direction of ligand exchange 配体交换 — which complex wins.
Stability constant lab
larger Kstab favours complex
Increase ligand binding strength and see complex formation become more complete.
The stability constant K_stab is:
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K_stab is the equilibrium constant for the complex-forming reaction; water is omitted.
The equilibrium constant for the formation of a complex from its aqua ion is the ______ constant.
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A larger value means a more stable complex.
A larger stability constant means the complex is more stable than the aqua ion.
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Ligand exchange is favoured.
What $K_{\text{stab}}$ tells you
- $K_{\text{stab}}$ is the equilibrium constant for forming a complex from the metal ion and its ligands (water is left out of the expression).
- A large $K_{\text{stab}}$ means a very stable complex.
Complex shapes by coordination number
A large K_stab means the complex is:
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A large stability constant indicates a strongly favoured, stable complex.
Direction of ligand exchange
- In a ligand exchange, the equilibrium shifts towards the complex with the larger $K_{\text{stab}}$.
- So a ligand that forms a more stable complex can push out a weaker one.
In a ligand exchange, the equilibrium shifts towards the complex with the:
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The more stable complex (larger K_stab) is favoured, so a stronger ligand can displace a weaker one.
The chelate effect 螯合效应
- A multidentate 多齿 (chelating) ligand forms a far more stable complex than several separate ligands.
- The reason is entropy 熵: swapping many small ligands for a few big ones increases the number of free particles.
[Ni(H₂O)₆]²⁺ + 3 en (a bidentate ligand) → [Ni(en)₃]²⁺ + 6 H₂O.
- 4 particles become 7 free particles — a big entropy rise.
- So $K_{\text{stab}}$ is huge, and the en complex easily displaces the water (or ammonia) ligands.
- This chelate effect is why EDTA (which grabs a metal ion at six points) binds so tightly.
You've got it
- $K_{\text{stab}}$ = equilibrium constant for forming a complex (water omitted)
- a large $K_{\text{stab}}$ = a very stable complex
- ligand exchange moves towards the complex with the larger $K_{\text{stab}}$
- the chelate effect: multidentate ligands give a bigger $K_{\text{stab}}$, driven by the entropy rise