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RO9. Electron Transfer: Inner Sphere

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:

\[ Co (NH_3)_6^{3+}   +   Cr^{2+}   \rightarrow   Co^{2+}   +   Cr^{3+}   +  6 NH_3   K = 10^{-4} M^{-1} s^{-1} \]

\[ Co (NH_3)_5 Cl^{2+}   +   Cr^{2+}   \rightarrow   Co^{2+}   +   CrCl^{2+}   +  6 NH_3   K = 6 \times 10^5 M^{-1} s^{-1} \]

(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. 

In the 1960’s, Henry Taube of Stanford University proposed that halides (and other ligands) may promote electron transfer via bridging effects.  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 1983; the award was based on his work on the mechanism of electron transfer reactions.

Problem RO9.1.

Take another look at the two electron transfer reactions involving the cobalt and chromium ion, above.

  1. What geometry is adopted by these complexes?
  2. Are these species high spin or low spin?
  3. Draw d orbital splitting diagrams for each complex.
  4. Explain why electron transfer is accompanied by loss of the ammonia ligands from the cobalt complex.
  5. The chloride is lost from the cobalt comples after electron transfer.  Why does it remain on the chromium?

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.

Problem RO9.2.

Draw an example of each of the ligands listed above bridging between a cobalt(III) and chromium(II) aqua complex.

 

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. 

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, becuase 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.