Inorganic — redox (electron transfer)
Inner-sphere electron transfer: Cr²⁺ + [Co(NH₃)₅Cl]²⁺
How the Inner-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: Cr²⁺(aq) + [Co(NH₃)₅Cl]²⁺. Conditions: Aqueous acid, 25 °C. Bonds formed: Cr–Cl bond; Cr(III); bonds broken: Co–Cl bond; Cr–OH₂. Stereochemistry: Chloride transferred from Co to Cr (Taube). Rate and selectivity: k ≈ 6×10⁵ M⁻¹s⁻¹; bridging ligand needed.
At a glance
Step 1: Cr(II) binds the bridging chloride
Labile Cr²⁺ displaces a water ligand and coordinates directly to the chloride still bound to the inert Co(III) center, forming a chloride-bridged binuclear intermediate.
Step 2: Electron transfer through the bridge
An electron transfers from Cr(II) to Co(III) through the bridging chloride — the defining feature of an inner-sphere mechanism (first characterized by Henry Taube).
Step 3: Bridge breaks; Cl stays on Cr(III)
The bridge collapses: chloride ends up on the now-inert Cr(III) product, while the newly formed, labile Co(II) center rapidly loses its remaining ligands.
Abbreviations and terms
- M
- Metal. A generic metal centre.
How it compares with related reactions
| Inner-sphere electron transfer | Outer-sphere electron transfer | Asymmetric phase-transfer catalysis | |
|---|---|---|---|
| Reagents | Cr²⁺(aq) + [Co(NH₃)₅Cl]²⁺ | [Fe(H₂O)₆]²⁺ and [Fe(H₂O)₆]³⁺ | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr |
| Conditions | Aqueous acid, 25 °C | Aqueous solution, 25 °C | Toluene/CH₂Cl₂, 0 °C, vigorous stirring |
| Stereochemistry | Chloride transferred from Co to Cr (Taube) | Fe–O bonds contract in Fe(III), lengthen in Fe(II) | High ee: chiral ion pair shields one enolate face |
| Rate / selectivity | k ≈ 6×10⁵ M⁻¹s⁻¹; bridging ligand needed | k ≈ 4 M⁻¹s⁻¹; Marcus reorganization energy | 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 Inner-sphere electron transfer?
Cr²⁺(aq) + [Co(NH₃)₅Cl]²⁺. Conditions: Aqueous acid, 25 °C.
Which bonds are formed and which are broken?
Formed: Cr–Cl bond; Cr(III). Broken: Co–Cl bond; Cr–OH₂.
What is the stereochemical outcome?
Chloride transferred from Co to Cr (Taube)
What controls the rate or the selectivity?
k ≈ 6×10⁵ M⁻¹s⁻¹; bridging ligand needed
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam