Inorganic — alkene π-complexation
Zeise's salt: alkene-metal π-complex formation
How the Zeise's salt 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], ethylene (H₂C=CH₂), cat. SnCl₂ optional. Conditions: Aqueous HCl, 25 °C, 1 atm C₂H₄. Bonds formed: Pt–(η²-C₂H₄) bonds (σ-donation + π-back-donation); bonds broken: Pt–Cl (one Cl⁻ displaced). Stereochemistry: Ethylene perpendicular to the PtCl₃ plane; C=C lengthens to ~1.37 Å. Rate and selectivity: Fast ligand substitution at square-planar Pt(II).
At a glance
Step 1: π-donation: alkene → Pt
An alkene's filled π orbital donates electron density into an empty d-orbital on platinum(II) — ordinary Lewis base-to-acid donation, the same electronic principle behind every metal-alkene coordination covered throughout this library's organometallic entries.
Step 2: π back-donation: Pt → alkene
Simultaneously, a filled d-orbital on platinum donates electron density back into the alkene's empty π* antibonding orbital — this back-bonding is what distinguishes true organometallic π-complexation from simple ionic or dative bonding, and it's what weakens and activates the alkene toward the nucleophilic attack seen in Wacker oxidation and aza-Wacker cyclization.
Step 3: Zeise’s salt
The result, [PtCl₃(C₂H₄)]⁻ (Zeise's salt), was the first organometallic compound ever characterized, and this donation/back-donation picture — now called the Dewar-Chatt-Duncanson model — is the conceptual foundation underlying essentially every alkene-activation step used throughout this library's Pd- and Pt-catalyzed reactions.
Abbreviations and terms
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
- π*
- Pi antibonding orbital. The empty orbital of a C=O or C=C that accepts nucleophiles.
How it compares with related reactions
| Zeise's salt | Associative ligand substitution at a square-planar Pt(II) complex | Asymmetric phase-transfer catalysis | |
|---|---|---|---|
| Reagents | K₂[PtCl₄], ethylene (H₂C=CH₂), cat. SnCl₂ optional | K₂[PtCl₄] + NH₃ (aq) | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr |
| Conditions | Aqueous HCl, 25 °C, 1 atm C₂H₄ | H₂O, 25 °C | Toluene/CH₂Cl₂, 0 °C, vigorous stirring |
| Stereochemistry | Ethylene perpendicular to the PtCl₃ plane; C=C lengthens to ~1.37 Å | Trigonal-bipyramidal intermediate; retention of geometry | High ee: chiral ion pair shields one enolate face |
| Rate / selectivity | Fast ligand substitution at square-planar Pt(II) | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) | Alkylation of the ion pair is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Zeise's salt?
K₂[PtCl₄], ethylene (H₂C=CH₂), cat. SnCl₂ optional. Conditions: Aqueous HCl, 25 °C, 1 atm C₂H₄.
Which bonds are formed and which are broken?
Formed: Pt–(η²-C₂H₄) bonds (σ-donation + π-back-donation). Broken: Pt–Cl (one Cl⁻ displaced).
What is the stereochemical outcome?
Ethylene perpendicular to the PtCl₃ plane; C=C lengthens to ~1.37 Å
What controls the rate or the selectivity?
Fast ligand substitution at square-planar Pt(II)
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam