Substitution in Octahedral Complexes
Concept Overview
Ligand substitution in 6-coordinate octahedral complexes (primarily and ) proceeds almost exclusively via Dissociative (D) or Interchange-dissociative (Id) mechanisms.
The substitution rates are controlled by six sub-factors, explored in the interactive dashboard below.
Key Equations
- — equilibrium constant for the deprotonation pre-equilibrium
- — rate constant for RDS dissociation of the conjugate base
Worked Examples
Explore each of the six factors controlling octahedral substitution in the interactive dashboard below.
Common Misconceptions
❌ Misconception
Since base hydrolysis rate depends on [OH⁻], the mechanism must be an associative (SN2) attack by hydroxide.
✅ Correction
The mechanism is strictly dissociative (SN1cb). OH⁻ acts as a base to deprotonate a ligand, not as a nucleophile. [OH⁻] dependence comes from the pre-equilibrium.
Interactive Visual
Explore the six factors that control octahedral substitution rates:
1. Leaving Group
The weaker the M–X bond, the faster the reaction. Rate increases down the halogen group: F⁻ ≪ Cl⁻ < Br⁻ < I⁻. This is because the bond dissociation energy decreases as the halide radius increases.