File Name: inner sphere and outer sphere mechanism .zip
These metrics are regularly updated to reflect usage leading up to the last few days.
In some cases, electron transfers occur much more quickly in the presence of certain ligands. For example, compare the rate constants for the following two electron transfer reactions, involving almost exactly the same complexes:.
Note: aqua ligands are omitted for simplicity. Ions, unless noted otherwise, are aqua complexes. Notice two things: first, when there is a chloride ligand involved, the reaction is much faster.
Second, after the reaction, the chloride ligand has been transferred to the chromium ion. Possibly, those two events are part of the same phenomenon. Similar rate enhancements have been reported for reactions in which other halide ligands are involved in the coordination sphere of one of the metals.
What he meant was that the chloride ion could use one of its additional lone pairs to bind to the chromium ion. It would then be bound to both metals at the same time, forming a bridge between them. Perhaps the chloride could act as a conduit for electron transfer. The chloride might then remain attached to the chromium, to which it had already formed a bond, leaving the cobalt behind. Electron transfers that occur via ligands shared by the two metals undergoing oxidation and reduction are termed "inner sphere" electron transfers.
Taube was awarded the Nobel Prize in chemistry in ; the award was based on his work on the mechanism of electron transfer reactions. Take another look at the two electron transfer reactions involving the cobalt and chromium ion, above.
Other ligands can be involved in inner sphere electron transfers. These ligands include carboxylates, oxalate, azide, thiocyanate, and pyrazine ligands. All of these ligands have additional lone pairs with which to bind a second metal ion. Draw an example of each of the ligands listed above bridging between a cobalt III and chromium II aqua complex.
Explain, with structures and d orbital splitting diagrams, how the products are formed in the following reaction, in aqueous solution. Once the bridge is in place, the electron transfer may take place via either of two mechanisms. Suppose the bridging ligand is a chloride. The first step might actually involve an electron transfer from chlorine to the metal; that is, the chloride could donate one electron from one of its idle lone pairs.
This electron could subsequently be replaced by an electron transfer from metal to chlorine. Sometimes, we talk about the place where an electron used to be , describing it as a "hole".
In this mechanism, the electron donated from the bridging chloride ligand leaves behind a hole. The hole is then filled with an electron donated from the other metal. Alternatively, an electron might first be transferred from metal to chlorine, which subsequently passes an electron along to the other metal.
In the case of chlorine, this idea may be unsatisfactory, because chlorine already has a full octet. Nevertheless, some of the other bridging ligands may have low-lying unoccupied molecular orbitals that could be populated by this extra electron, temporarily. One of the many contributions to the barrier for electron transfer between metal ions is internal electronic reorganization.
The need to move electrons between different d orbitals on the cobalt will add to the barrier, slowing down the reaction. Furthermore, the conjugation in the bridging ligand would help in conducting an electron from one end of the ligand to the other, either through an electron mechanism or a hole mechanism.
One potential problem in measuring rates of intramolecular electron transfer i. If the rates were the same across a number of different concentrations, the reaction would probably be intramolecular. Stephan Isied and coworkers at Rutgers measured the following electron transfer rates between metal centers separated by a peptide. Chem Rev , 92 , Is your plot linear? Isied offers a number of possible explanations for the data, all of which involve two competing reaction pathways.
If the molecule can't wiggle around as much, then the distance between the ends of the molecule should be more constant. There are few pi bonds or lone pairs to use as places to put electrons or temporarily remove electrons from, shuttling the electrons from place to place along the ligand.
A conjugated system would be much more likely to carry out inner sphere electron transfer. That's a classic symptom of two competing mechanisms. The faster mechanism, to the left, is probably an intramolecular electron transfer. The slower mechanism, to the right, may be an intermolecular electron transfer. Chris P Schaller, Ph. What geometry is adopted by these complexes? Are these species high spin or low spin? Draw d orbital splitting diagrams for each complex.
Explain why electron transfer is accompanied by loss of the ammonia ligands from the cobalt complex. The chloride is lost from the cobalt complex after electron transfer. Why does it remain on the chromium? The ligands are easily replaced by water.
Answer c c The pathway is probably inner sphere because of the bridging ligand. Answer a a The rate decreases exponentially as distance increases.
Answer b b You might keep the concentration low in order to increase the distance between molecules, reducing the likely hood of an outer-sphere electron transfer. Answer c c If you ran the experiment at a series of dilutions, intramolecular electron transfer would be unaffected but outer sphere electron transfer would not.
Chem Rev , 92 , The proline repeating unit is crucial in ensuring a steady increase in distance between metal centers with increased repeat units, n. An inner sphere pathway in this case is expected to be somewhat slow because of the lack of conjugation in the polyproline bridge.
Explain why. Plot the data below, with log k on the y axis range from and d on the x axis Angstroms. Attribution Chris P Schaller, Ph.
In some cases, electron transfers occur much more quickly in the presence of certain ligands. For example, compare the rate constants for the following two electron transfer reactions, involving almost exactly the same complexes:. Note: aqua ligands are omitted for simplicity. Ions, unless noted otherwise, are aqua complexes. Notice two things: first, when there is a chloride ligand involved, the reaction is much faster. Second, after the reaction, the chloride ligand has been transferred to the chromium ion.
These are preliminary reports that have not been peer-reviewed. For more information, please see our FAQs. Thumbnail List Side list File only.
Electron-transfer reactions are reactions of the oxidative-reductive type. They are of great importance in nature, but also in technology, and have been in the center of interest during the last 50 years, which is evident from several thousands of publications in the field. Many biological processes, e.
Outer sphere refers to an electron transfer ET event that occurs between chemical species that remain separate and intact before, during, and after the ET event.
Your email address will not be published. Required fields are marked *