Inorganic — coordination chemistry
Associative ligand substitution at a square-planar Pt(II) complex
How the Associative ligand substitution at a square-planar Pt(II) 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: K₂[PtCl₄] + NH₃ (aq). Conditions: H₂O, 25 °C. Bonds formed: Pt–N dative bond; bonds broken: Pt–Cl. Stereochemistry: Trigonal-bipyramidal intermediate; retention of geometry. Rate and selectivity: Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent).
At a glance
Step 1: NH₃ attacks the 16 e⁻ metal
[PtCl₄]²⁻ is a 16-electron, coordinatively unsaturated square-planar complex — there's room for one more ligand. NH₃'s lone pair attacks the electrophilic metal center from above or below the plane.
Step 2: Five-coordinate intermediate
This forms a five-coordinate, trigonal-bipyramidal intermediate — the defining feature of an associative (A) mechanism, common for d⁸ square-planar metals like Pt(II), Pd(II), and Au(III).
Step 3: Chloride leaves
A chloride ligand leaves from the opposite face, restoring four-coordination and giving the substituted complex [PtCl₃(NH₃)]⁻.
How it compares with related reactions
| Associative ligand substitution at a square-planar Pt(II) complex | Dissociative ligand substitution at an octahedral complex | CuAAC | |
|---|---|---|---|
| Reagents | K₂[PtCl₄] + NH₃ (aq) | [Co(NH₃)₅Cl]²⁺ + H₂O (entering ligand Y) | Terminal alkyne, organic azide, Cu(I) (CuSO₄ + sodium ascorbate, or CuI), base/ligand (TBTA) |
| Conditions | H₂O, 25 °C | Aqueous solution, 25 °C | tBuOH/H₂O or DMSO, 25 °C |
| Stereochemistry | Trigonal-bipyramidal intermediate; retention of geometry | Often retention (square-pyramidal intermediate) | 1,4-regioisomer only (thermal gives 1,4 + 1,5 mixture) |
| Rate / selectivity | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) | Rate = k[complex]; independent of [Y] (D / Id) | 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 Associative ligand substitution at a square-planar Pt(II) complex?
K₂[PtCl₄] + NH₃ (aq). Conditions: H₂O, 25 °C.
Which bonds are formed and which are broken?
Formed: Pt–N dative bond. Broken: Pt–Cl.
What is the stereochemical outcome?
Trigonal-bipyramidal intermediate; retention of geometry
What controls the rate or the selectivity?
Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent)
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam