Inorganic — organometallic
σ-bond metathesis at an early transition metal
How the σ-bond metathesis at an early transition metal 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: d⁰ metal alkyl (Cp*₂Zr–CH₃, Cp*₂Sc–CH₃, Cp*₂Lu–CH₃) + H₂. Conditions: Hydrocarbon solvent, 25 °C. Bonds formed: M–H; C–H (alkane); bonds broken: M–C; H–H. Stereochemistry: Retention at carbon; four-centre kite TS. Rate and selectivity: No oxidation-state change; metal stays d⁰ (Zr(IV)).
At a glance
Step 1: d⁰ metal alkyl meets H₂
An electron-poor, coordinatively unsaturated early metal alkyl (e.g., a d⁰ Zr complex) approaches H₂ — too electron-poor to do oxidative addition, which needs d-electrons to donate.
Step 2: Four-centre transition state
Instead, the M–C and H–H bonds interact through a four-centered transition state, exchanging partners directly without any change in the metal's oxidation state.
Step 3: Zirconium hydride and methane
The result is a new M–H bond and free alkane, released in one concerted step — common for d⁰ metals like Zr(IV), Sc(III), and lanthanides.
Abbreviations and terms
- M
- Metal. A generic metal centre.
- Cp
- Cyclopentadienyl. A five-carbon aromatic ring ligand (C₅H₅⁻) that binds metals through all five carbons.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| σ-bond metathesis at an early transition metal | Cross metathesis between two different alkenes | Migratory insertion | |
|---|---|---|---|
| Reagents | d⁰ metal alkyl (Cp*₂Zr–CH₃, Cp*₂Sc–CH₃, Cp*₂Lu–CH₃) + H₂ | Ru carbene (Grubbs II), two different alkenes | LₙM–CH₃ (e.g. Mn(CO)₅CH₃) + CO |
| Conditions | Hydrocarbon solvent, 25 °C | CH₂Cl₂, 25–40 °C; excess partner or ethylene removal | CO atmosphere, 25–100 °C |
| Stereochemistry | Retention at carbon; four-centre kite TS | Usually E-selective (thermodynamic) | Alkyl migrates; stereochemistry at carbon retained |
| Rate / selectivity | No oxidation-state change; metal stays d⁰ (Zr(IV)) | 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 σ-bond metathesis at an early transition metal?
d⁰ metal alkyl (Cp*₂Zr–CH₃, Cp*₂Sc–CH₃, Cp*₂Lu–CH₃) + H₂. Conditions: Hydrocarbon solvent, 25 °C.
Which bonds are formed and which are broken?
Formed: M–H; C–H (alkane). Broken: M–C; H–H.
What is the stereochemical outcome?
Retention at carbon; four-centre kite TS
What controls the rate or the selectivity?
No oxidation-state change; metal stays d⁰ (Zr(IV))
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam