The complex ion [Fe(bipy)₅Br]³⁺ (bipy = 2,2′-bipyridine) undergoes substitution of the bromide ligand by water. Write the balanced reaction, describe the mechanism of substitution, and give the rate law for this reaction.
Answer
The reaction proceeds via a doubly associative (A₂) mechanism; rate = k[Fe(bipy)₅Br]³⁺[H₂O]
The correct answer is The reaction proceeds via a doubly associative (A₂) mechanism; rate = k[Fe(bipy)₅Br]³⁺[H₂O] — Note that the rate depends on the concentration of the complex and the incoming ligand, not on the leaving bromide.
Step-by-step solution
- Write the balanced equation: [Fe(bipy)₅Br]³⁺ + H₂O → [Fe(bipy)₅(H₂O)]³⁺ + Br⁻.
- Explain the mechanism: water first coordinates to the metal, forming a 7‑coordinate intermediate (addition), followed by departure of Br⁻ (dissociation).
- Identify the mechanism as doubly associative (A₂) because two ligands are added before one leaves.
- State the rate law: rate = k[Fe(bipy)₅Br]³⁺[H₂O]; if water is in large excess the rate simplifies to rate = k[Fe(bipy)₅Br]³⁺.
- Note that the rate depends on the concentration of the complex and the incoming ligand, not on the leaving bromide.
Common mistake: Assuming the substitution is dissociative or writing a rate law that depends on [Br⁻] instead of [H₂O].
Concept tested: Substitution mechanism and kinetics
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