9701 · 28.5
Stability constants, Kstab
The stability constant, Kstab, is a number that tells us how tightly ligands are bound to a central metal ion. A bigger number means a more stable, 'happier' complex that is less likely to break apart.
Need to know
What you need to know
- A large Kstab value indicates the equilibrium position is far to the right, meaning the complex is very stable and readily forms.
- A small Kstab value indicates the complex is unstable and will readily dissociate back into its constituent ions.
- The units of Kstab vary depending on the value of 'n' (the number of ligands) and must be derived for each specific reaction.
Explanation
The Complex Popularity Contest
- The stability constant, Kstab, measures how readily a complex forms. Its formula is Kstab = [MLₙ] / ([M][L]ⁿ), where a large value means the complex is very stable.
- Different ligands form complexes of varying stability. A ligand forming a complex with a higher Kstab is considered 'stronger' and will displace ligands that form less stable complexes.
- Complexes form step-by-step, with each step having a constant (K₁, K₂, etc.). The overall stability constant, βn, is the product of all the stepwise constants (βn = K₁ × K₂ ×...).
- By comparing Kstab values, you can predict which complex is more stable. The reaction will favour the formation of the complex with the much larger Kstab value.