Inorganic — coordination chemistry
Dissociative ligand substitution at an octahedral complex
How the Dissociative ligand substitution at an octahedral complex works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: [Co(NH₃)₅Cl]²⁺ + H₂O (entering ligand Y). Conditions: Aqueous solution, 25 °C. Bonds formed: Co–OH₂ bond; bonds broken: Co–Cl bond. Stereochemistry: Often retention (square-pyramidal intermediate). Rate and selectivity: Rate = k[complex]; independent of [Y] (D / Id).
At a glance
Step 1: Six-coordinate: no free site
Octahedral complexes already have all six coordination sites occupied — unlike square-planar Pt(II), there's no open site for an incoming ligand to attack first.
Step 2: Slow Co–Cl dissociation
A ligand leaves first, generating a five-coordinate intermediate. This step is rate-determining — the defining feature of a dissociative (D) mechanism.
Step 3: Fast capture by the entering ligand
The new ligand then fills the vacant site rapidly. Because the rate depends only on how fast the leaving ligand departs, the incoming ligand's identity barely affects reaction rate.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- CO
- Carbon monoxide (carbonyl ligand). σ-donor / strong π-acceptor ligand; also a C1 building block.
How it compares with related reactions
| Dissociative ligand substitution at an octahedral complex | Associative ligand substitution at a square-planar Pt(II) complex | CuAAC | |
|---|---|---|---|
| Reagents | [Co(NH₃)₅Cl]²⁺ + H₂O (entering ligand Y) | K₂[PtCl₄] + NH₃ (aq) | Terminal alkyne, organic azide, Cu(I) (CuSO₄ + sodium ascorbate, or CuI), base/ligand (TBTA) |
| Conditions | Aqueous solution, 25 °C | H₂O, 25 °C | tBuOH/H₂O or DMSO, 25 °C |
| Stereochemistry | Often retention (square-pyramidal intermediate) | Trigonal-bipyramidal intermediate; retention of geometry | 1,4-regioisomer only (thermal gives 1,4 + 1,5 mixture) |
| Rate / selectivity | Rate = k[complex]; independent of [Y] (D / Id) | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) | 10⁷–10⁸ × faster than uncatalysed Huisgen cycloaddition |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Dissociative ligand substitution at an octahedral complex?
[Co(NH₃)₅Cl]²⁺ + H₂O (entering ligand Y). Conditions: Aqueous solution, 25 °C.
Which bonds are formed and which are broken?
Formed: Co–OH₂ bond. Broken: Co–Cl bond.
What is the stereochemical outcome?
Often retention (square-pyramidal intermediate)
What controls the rate or the selectivity?
Rate = k[complex]; independent of [Y] (D / Id)
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam