Inorganic — redox (electron transfer)
Outer-sphere electron transfer: Fe(H₂O)₆²⁺/³⁺ self-exchange
How the Outer-sphere electron transfer 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: [Fe(H₂O)₆]²⁺ and [Fe(H₂O)₆]³⁺. Conditions: Aqueous solution, 25 °C. Bonds formed: Swapped oxidation states; bonds broken: None (no ligand exchange). Stereochemistry: Fe–O bonds contract in Fe(III), lengthen in Fe(II). Rate and selectivity: k ≈ 4 M⁻¹s⁻¹; Marcus reorganization energy.
At a glance
Step 1: Intact complexes meet
Both complexes keep their full coordination spheres intact — no ligands bridge or exchange. They simply diffuse close enough for orbital overlap.
Step 2: Electron tunnels across the gap
An electron tunnels directly between the metal centers across the gap between them, with no bond breaking or forming on either complex.
Step 3: Complexes separate, states swapped
The complexes separate having swapped oxidation states. Rates here are governed by Marcus theory — the barrier comes from reorganizing solvent and bond lengths, not from breaking bonds.
Abbreviations and terms
- M
- Metal. A generic metal centre.
How it compares with related reactions
| Outer-sphere electron transfer | Inner-sphere electron transfer | Asymmetric phase-transfer catalysis | |
|---|---|---|---|
| Reagents | [Fe(H₂O)₆]²⁺ and [Fe(H₂O)₆]³⁺ | Cr²⁺(aq) + [Co(NH₃)₅Cl]²⁺ | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr |
| Conditions | Aqueous solution, 25 °C | Aqueous acid, 25 °C | Toluene/CH₂Cl₂, 0 °C, vigorous stirring |
| Stereochemistry | Fe–O bonds contract in Fe(III), lengthen in Fe(II) | Chloride transferred from Co to Cr (Taube) | High ee: chiral ion pair shields one enolate face |
| Rate / selectivity | k ≈ 4 M⁻¹s⁻¹; Marcus reorganization energy | k ≈ 6×10⁵ M⁻¹s⁻¹; bridging ligand needed | 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 Outer-sphere electron transfer?
[Fe(H₂O)₆]²⁺ and [Fe(H₂O)₆]³⁺. Conditions: Aqueous solution, 25 °C.
Which bonds are formed and which are broken?
Formed: Swapped oxidation states. Broken: None (no ligand exchange).
What is the stereochemical outcome?
Fe–O bonds contract in Fe(III), lengthen in Fe(II)
What controls the rate or the selectivity?
k ≈ 4 M⁻¹s⁻¹; Marcus reorganization energy
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam