What is the Difference Between Inner and Outer Sphere Mechanism?
🆚 Go to Comparative Table 🆚The inner and outer sphere mechanisms are two different types of electron transfer processes that occur in coordination complexes. The main differences between them are:
- Participation of ligands: In the inner sphere mechanism, electron transfer occurs between complexes via a bridging ligand, and at least one of the complexes needs to be labile to allow the bridge to form. In contrast, the outer sphere mechanism occurs between complexes that do not undergo substitution, and no new bonds are broken or formed. Bridging ligands are not involved in this mechanism.
- Bond breaking and formation: In the inner sphere mechanism, bonds are broken and formed during the electron transfer process. However, in the outer sphere mechanism, no new bonds are broken or formed.
- Speed: Outer sphere electron transfer is generally faster than inner sphere electron transfer because the energetic demands are less.
- Reactivity: The inner sphere mechanism is important when atom transfer (bond breaking/formation) can occur, while the outer sphere mechanism is an alternative pathway for the inner sphere mechanism.
In summary, the inner sphere mechanism involves the participation of ligands, bond breaking and formation, and is generally slower than the outer sphere mechanism, which occurs between complexes without substitution and does not involve ligands.
Comparative Table: Inner vs Outer Sphere Mechanism
The difference between inner and outer sphere mechanisms lies in the process of electron transfer and the involvement of ligands. Here is a table comparing the two mechanisms:
| Inner Sphere Mechanism | Outer Sphere Mechanism |
|---|---|
| Occurs via a covalent bond or linkage | Occurs between two separate species |
| Involves a ligand that bridges the oxidant and reductant reactants during the reaction | Reactants and products exist separate from each other before, during, and after electron transfer |
| Large ligands can inhibit the reaction | No bridging between reactants in outer sphere mechanism |
| Rare in biological systems due to large ligands preventing the formation of crucial bridged complexes | More common in biological systems and faster because the energetic demands are less |
| Reactions involve the participation of the inner sphere (closest molecules of the solvent shell or ligands in complexes) | Reactions do not involve the inner sphere, only an electron transfer takes place |
In summary, the inner sphere mechanism involves a covalent bond or linkage and requires a bridging ligand, while the outer sphere mechanism occurs between separate chemical species without any bridging. The inner sphere mechanism is less common in biological systems due to the inhibiting effect of large ligands, while the outer sphere mechanism is more common and faster.
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