Frequently Asked Question

The aquation of cobalt(III) complexes, where a water molecule replaces a ligand, typically follows an associative (A) mechanism. Here's how the mechanism explains the observed dependencies:
1. Leaving Group:
A good leaving group (weakly bound) readily departs, facilitating the reaction. The rate increases with better leaving groups.
2. Size of Non-Leaving Groups:
Larger non-leaving groups sterically hinder the approach of the incoming water molecule, slowing down the reaction. This is consistent with the observed rate dependence on the size of non-leaving groups.
3. Charge on the Complex:
A more positively charged complex attracts the incoming water molecule more strongly due to electrostatic interactions, accelerating the reaction. Conversely, a negatively charged complex would experience weaker attraction, leading to a slower rate.
Mechanism:
The mechanism involves a two-step process:
1. Formation of an intermediate: The water molecule enters the coordination sphere of the cobalt(III) complex, forming a seven-coordinate intermediate. This step is often the rate-determining step.
2. Dissociation of the leaving group: The leaving group departs, generating the aquated product. This step usually occurs rapidly.
Key Points:

The A mechanism involves the formation of an intermediate with higher coordination number, followed by the departure of the leaving group.
The rate-determining step is typically the formation of the intermediate, explaining the observed dependencies on leaving group, non-leaving group size, and complex charge.