In this class of electron transfer reaction, a ligand–bridged intermediate is formed. Proof that such intermediates were involved was obtained by Carol Creutz, Henry Taube and their co–workers. Taube won the Nobel prize in the early 1980’s for this. For example, consider the reaction (VI), done in acid conditions.
[Co(NH3)5Cl]2+ + [Cr(H2O)6]2+ ® [Cr(H2O)5Cl]2+ + [Co(H2O)6]2+ + 5 NH4+ (VI)
The first thing that is worth noting about this reaction is that, if it proceeded by an outer-sphere reaction mechanism, we would expect the rate to be slow, because the two electron self-exchange processes involved, k1 and k2 in the Marcus equation, i.e. Co(II)/Co(III), and Cr(II)/Cr(III), will themselves be very slow (the Co(II)/Co(III) case we have already discussed, but there is also a large bond length difference expected for Cr(II)/Cr(III) because Cr(II) is high-spin d4 whereas Cr(III) is d3).
It is noted that (1) the observed rate constant k12 for this reaction is much faster than that predicted by the Marcus equation, and (2) unlike outer-sphere mechanisms, a ligand seems to have ‘changed places’ in the course of the reaction.
Co(III) complexes are extremely inert to ligand substitution because they are low spin d6. Cr(II) complexes are very labile (high spin d4). On the right hand side, Cr(III) complexes are also inert, but Co(II) complexes are labile (high spin d7). The Cr(III) complex could be isolated from the reaction because it is inert, and proof obtained that it has a Cl– ligand, although the Cr(II) reactant did not. If the reaction is performed in the presence of free radiolabelled Cl– ions, no radiolabelled Cl– ends up coordinated to the Cr(III), proving that the Cl– ligand must have originated from the Co(III) complex. The Co(II)–ammine complex generated decomposes in the presence of acid, giving the hexaaquo ion as shown. The separate steps in the process are:
Substitution of one water ligand on the labile Cr(II) ion by a second lone pair on the Cl ligand from the Co(III) complex:
[Cr(H2O)6]2+ +[Co(NH3)5Cl]2+ ® [(H2O)5CrII–(m–Cl)–CoIII(H2O)5]4+ + H2O
Electron transfer from Cr(II) to Co(III) over the Cl– bridge:
[(H2O)5CrII–Cl–CoIII(NH3)5]4+ ® [(H2O)5CrIII–Cl–CoII(NH3)5]4+
Hydrolysis of the now labile Co(II)–Cl bond:
[(H2O)5CrIII–Cl–CoII(NH3)5]4+ ® [(H2O)5CrIII–Cl]2+ + [CoII(NH3)5(H2O)]2+
Finally, the Co(II)–ammine complex, now labile to ligand substitution, hydrolyses rapidly under the acidic conditions to give [Co(H2O)6]2+ and 5 NH4+
It is interesting to note that electron transfer reactions can sometimes be used in synthesis. One brief example will now be given.
The thiocyanate ion NCS– is ambidentate. Usually, 3d metal ions prefer to bond to the nitrogen, M–NCS giving an N–thiocyanato complex. For example,
[Cr(H2O)6]3+ + NCS– ® [Cr(NCS)(H2O)5]2+ (very slow!)
However, by using a redox reaction exactly like the one above, the Cr–SCN (S–thiocyanato) complex can be made, because the thiocyanate ion can bridge by bonding to the Cr(II) via the sulfur:
[Cr(H2O)6]2+ + [SCN–Co(NH3)5]2+ ® [NCS–Cr(H2O)5] + [Co(H2O)6]2+ + 5 NH4+