The complex ion [Co(phen)3]²⁺ (phen = 1,10‑phenanthroline, bidentate) undergoes substitution of one phen ligand by a water molecule to give [Co(phen)2(H₂O)₂]²⁺. Write the balanced substitution reaction, describe the substitution mechanism, and give the rate law for this reaction.
Answer
Substitution: [Co(phen)3]²⁺ + 2 H₂O → [Co(phen)2(H₂O)2]²⁺ + 2 phen. Mechanism: dissociative (d). Rate law: rate = k[Co(phen)3]²⁺.
Step-by-step solution
- Step 1: Identify leaving ligand – one phen (bidentate) departs as a neutral ligand.
- Step 2: Two water molecules coordinate to fill the two vacant sites left by the departing phen. The overall equation: [Co(phen)3]²⁺ + 2 H₂O → [Co(phen)2(H₂O)2]²⁺ + 2 phen.
- Step 3: In a d‑mechanism, the complex first loses a ligand to form a 5‑coordinate intermediate; the water then inserts. The rate‑determining step is ligand dissociation, so rate = k[Co(phen)3]²⁺.
- Step 4: The reaction is zero‑order in water because the dissociation of the metal complex is rate‑limiting.
Common mistake: Assuming associative mechanism or incorrect ligand stoichiometry
Concept tested: Substitution mechanism in coordination complexes
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