Complex ion formation constants – stepwise ligand binding
ChemistryInorganic ChemistryAges 17–18
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Inorganic Chemistry
A ligand does not replace all the water molecules at once; it attaches to the metal ion one step at a time, each step with its own stepwise formation constant (also called stability constant). Drag the ligand concentration to watch the species-distribution diagram for [M(H₂O)ₙ₋ᵢLᵢ] shift, the solution color change with it, and compare NH₃ with ethylenediamine to see the chelate effect make a complex millions of times more stable.
Lesson: Complex ion equilibria and formation constants
What it shows
Ligands replace the water molecules around a hydrated metal ion one at a time, so a solution usually contains several complexes at once. Each step has a stepwise formation constant Kᵢ, and the overall constant βₙ is their product, so log βₙ is the sum of the log Kᵢ values. The share of each species depends on the free ligand concentration, as a species-distribution diagram shows. Bidentate ethylenediamine forms five-membered chelate rings and gives far more stable complexes than NH₃ with the same number of nitrogen donors, largely because entropy increases.
How to use
Choose the Central ion and the Ligand, then drag log[free ligand] from very dilute towards concentrated. Watch which complex dominates at each value, read the percentage of each species and the average number of bound ligands, and note the solution color. Switch from NH₃ to ethylenediamine with the same metal ion to see the curves shift to much lower ligand concentrations.
Parameters you can change
- Central ion Cu²⁺, Ni²⁺, Zn²⁺
- Ligand NH₃ (monodentate), Ethylenediamine (bidentate)
- log[free ligand] -12–0
Questions to explore
- Why does a Cu²⁺ solution contain a mixture of several ammine complexes at intermediate NH₃ concentrations?
- How is the overall formation constant βₙ related to the stepwise constants K₁, K₂, K₃ and so on?
- What is the chelate effect, and how much more stable is the ethylenediamine complex of Ni²⁺ than its NH₃ complex?
