Inorganic — organometallic
Oxidative addition of methyl iodide to Pt(0)
How the Oxidative addition of methyl iodide to Pt(0) 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: Pt(PPh₃)₂ (14 e⁻ Pt(0)) + CH₃I. Conditions: Benzene or THF, 25 °C. Bonds formed: Pt–C and Pt–I σ bonds; bonds broken: C–I σ bond. Stereochemistry: Concerted 3-centre: cis addition; SN2-type with polar substrates. Rate and selectivity: Faster with electron-rich metals, I > Br > Cl, less hindered R.
At a glance
Step 1: Filled d orbital attacks carbon
Pt(PPh₃)₂ is electron-rich and coordinatively unsaturated. A filled metal d-orbital attacks the electrophilic carbon of CH₃–I, the way a nucleophile attacks any electrophilic carbon.
Step 2: Three-centre transition state
The C–I bond breaks as new Pt–C and Pt–I bonds form simultaneously. The metal's formal oxidation state rises by two (Pt(0) → Pt(II)) and its coordination number rises by two — both atoms of the substrate are now bound to the metal.
Step 3: Pt(II) alkyl iodide
The product, a four-coordinate Pt(II) complex bearing both methyl and iodide ligands, is the classic first step of many catalytic cycles (e.g., in cross-coupling reactions).
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- PPh3
- Triphenylphosphine. Nucleophilic phosphine; forms very strong P=O bonds, driving Wittig, Appel and Mitsunobu reactions.
- σ*
- Sigma antibonding orbital. The empty orbital an incoming nucleophile attacks in SN2 (the back lobe of C–LG).
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
How it compares with related reactions
| Oxidative addition of methyl iodide to Pt(0) | Cross metathesis between two different alkenes | Migratory insertion | |
|---|---|---|---|
| Reagents | Pt(PPh₃)₂ (14 e⁻ Pt(0)) + CH₃I | Ru carbene (Grubbs II), two different alkenes | LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO |
| Conditions | Benzene or THF, 25 °C | CH₂Cl₂, 25–40 °C; excess partner or ethylene removal | CO atmosphere, 25–100 °C |
| Stereochemistry | Concerted 3-centre: cis addition; SN2-type with polar substrates | Usually E-selective (thermodynamic) | Alkyl migrates; stereochemistry at carbon retained |
| Rate / selectivity | Faster with electron-rich metals, I > Br > Cl, less hindered R | Initiation/phosphine-or-ether release often limits; [2+2] steps fast | Alkyl must be cis to CO; ligand then fills the vacant site |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Oxidative addition of methyl iodide to Pt(0)?
Pt(PPh₃)₂ (14 e⁻ Pt(0)) + CH₃I. Conditions: Benzene or THF, 25 °C.
Which bonds are formed and which are broken?
Formed: Pt–C and Pt–I σ bonds. Broken: C–I σ bond.
What is the stereochemical outcome?
Concerted 3-centre: cis addition; SN2-type with polar substrates
What controls the rate or the selectivity?
Faster with electron-rich metals, I > Br > Cl, less hindered R
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam