Inorganic — thermodynamic/kinetic effect
The chelate effect in ligand binding
How the The chelate effect in ligand binding works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: [Ni(H₂O)₆]²⁺, ethylenediamine (en) vs 2 NH₃. Conditions: Water, 25 °C. Bonds formed: two Ni–N bonds (5-membered chelate ring); bonds broken: Ni–OH₂ (two). Stereochemistry: Ring closure gives a puckered five-membered ring. Rate and selectivity: log K₁(en) ≈ 7.5 vs log β₂(NH₃) ≈ 5.0 for Ni²⁺: entropy-driven.
At a glance
Step 1: First N binds (ordinary substitution)
A bidentate ligand's first donor atom coordinates to the metal exactly as any monodentate ligand would — no new bonding chemistry yet, just an ordinary associative substitution.
Step 2: Tethered second N closes the ring
Because the second donor atom is already tethered nearby, its subsequent coordination is far more favorable entropically than a separate free ligand finding the metal from bulk solution — this intramolecular step barely costs any translational entropy.
Step 3: Why the ring wins: entropy
The result is a much larger overall binding constant for the chelating ligand versus two separate monodentate ligands with identical donor atoms — the same mechanism (sequential ligand substitution) is at work, but favorable entropy alone drives dramatically enhanced stability.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
How it compares with related reactions
| The chelate effect in ligand binding | The trans effect in square-planar substitution | Associative ligand substitution at a square-planar Pt(II) complex | |
|---|---|---|---|
| Reagents | [Ni(H₂O)₆]²⁺, ethylenediamine (en) vs 2 NH₃ | K₂[PtCl₄], NH₃ (cisplatin route); [Pt(NH₃)₄]²⁺, Cl⁻ (transplatin route) | K₂[PtCl₄] + NH₃ (aq) |
| Conditions | Water, 25 °C | Water, 25–50 °C | H₂O, 25 °C |
| Stereochemistry | Ring closure gives a puckered five-membered ring | Ligand order of addition controls cis vs trans | Trigonal-bipyramidal intermediate; retention of geometry |
| Rate / selectivity | log K₁(en) ≈ 7.5 vs log β₂(NH₃) ≈ 5.0 for Ni²⁺: entropy-driven | Trans-effect series: CN⁻, CO, C₂H₄ > PR₃ > H⁻ > CH₃⁻ > I⁻ > Cl⁻ > NH₃ > H₂O | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the The chelate effect in ligand binding?
[Ni(H₂O)₆]²⁺, ethylenediamine (en) vs 2 NH₃. Conditions: Water, 25 °C.
Which bonds are formed and which are broken?
Formed: two Ni–N bonds (5-membered chelate ring). Broken: Ni–OH₂ (two).
What is the stereochemical outcome?
Ring closure gives a puckered five-membered ring
What controls the rate or the selectivity?
log K₁(en) ≈ 7.5 vs log β₂(NH₃) ≈ 5.0 for Ni²⁺: entropy-driven
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam