Why do chelating polydentate ligands form coordination complexes that are substantially more thermodynamically stable than those formed by similar monodentate ligands (the chelate effect)?
Answer and explanation
Correct answer: A. Favorable positive entropy change () from releasing coordinated solvent molecules
- Step 1: Consider the displacement reaction: .
- Step 2: In this process, 4 reacting particles produce 7 independent particles in solution, resulting in an increase in translational degrees of freedom.
- Step 3: This substantial positive entropy of reaction () renders much more negative, creating high thermodynamic stability.
Answer
Chelation is driven by a large positive entropy change () as multiple monodentate solvent molecules are released.
Why not the others: Chelating ligands always form stronger covalent bonds is inaccurate because the primary driving force is entropic rather than purely enthalpic; Chelation occurs with a huge decrease in entropy is thermodynamically false; Chelating complexes are always diamagnetic is false because magnetic properties depend on -electron count.
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