A New Face - Oxidation-Reduction

Oxidation-reduction (electron transfer) reactions are important in chemistry and biology. When a chemical oxidation of A by B occurs, B itself is reduced - an electron transfer process has occurred. For such chemical processes, there is always a partnership between an oxidant (which is reduced in carrying out its task) and a reductant (which is oxidized in the reaction) thus we frequently talk of oxidation-reduction, or (for ease of use) redox, reactions for what are essentially electron transfer processes. Of course, an electron can be 

A reaction involving electron transfer alone. The Fe(II) centre is oxidized to Fe(III) and the Ir(IV) centre is reduced to Ir(III), without any change in the ligand set for either complex. 

This reaction of the two octahedral complexes occurs without any change in the coordination spheres, or ligand sets, of either metal. However, if you inspect the two metal centres, you will note that the iron complex (the reductant) is oxidized from Fe(II) to Fe(III) and at the same time the iridium complex (the oxidant) is reduced from Ir(IV) to Ir(III) - an electron has been transferred from one metal to the other. This reaction can be conveniently followed, since the colour of each species changes as it is converted from one oxidation state to another. 

Atom transfer alone can also occur, although it is a more difficult concept to come to grips with. Think of it as an atom moving with its normal complement of valence electrons, and no others, from one central atom to another. Oxidative addition reactions in organometallic chemistry can be considered as a form of atom transfer. A typical example of this type is given in Equation 5.53. 

In the above example, an electron is transferred from the Cr(II) to the Co(III) and a chloride ion is transferred from the cobalt to the chromium ion as well. The Cr(III) product is inert, allowing it to be isolated and identified, thereby defining the presence of the chloride ion in its coordination sphere. Colour changes in this reaction allow the reaction to be readily monitored spectrophotometrically. 

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