Frequently Asked Question

The aquation of octahedral cobalt(III) complexes likely proceeds through a dissociative (D) mechanism. Here's a breakdown of the mechanism and how it aligns with the observations:
Mechanism:
1. Dissociation: The cobalt(III) complex loses a leaving group (L) to form a five-coordinate intermediate. This step is the rate-determining step and is consistent with the observation that the rate of aquation is dependent on the nature of the leaving group.
[Co(III)L(NH3)5]^(n+) ⇌ [Co(III)(NH3)5]^(n+3) + L
2. Association: The five-coordinate intermediate rapidly reacts with a water molecule to form the aquated product.
[Co(III)(NH3)5]^(n+3) + H2O → [Co(III)(H2O)(NH3)5]^(n+3)
Consistency with Observations:

Leaving Group: The rate of aquation is faster for better leaving groups. This is because the dissociation step, which involves the departure of the leaving group, becomes more favorable with a better leaving group.
Size of Non-Leaving Groups: Larger non-leaving groups can sterically hinder the formation of the five-coordinate intermediate, slowing down the aquation process.
Charge on the Complex: A higher positive charge on the complex increases the attraction between the metal ion and the leaving group, making it harder to break the bond and thus slowing down the aquation reaction.
Important Note: While the D mechanism is generally accepted for cobalt(III) complexes, some cases may involve an associative (A) or an interchange (I) mechanism depending on the specific ligands and reaction conditions.