Ligand effects on redox potentials of coordination compounds

11 Ligand effects on redox potentials of coordination compounds 

In 1898, Peters45 observed the effects of complex formation on the Fe(II)/Fe(III) redox potential. He found that, in aqueous solution, the redox potential of a system consisting of iron(III) and iron(II) ions depended on the concentration of hydrochloric acid. Carter and Clews,46 in 1924, explained this phenomenon in terms of metal ion complexation. Michaelis and coworkers47,48 carried out a systematic study of the influence of anions on the 

Fe(II)/Fe(IID redox potential. They showed that replacement of one anion by another resulted in potential changes over the range +0.7 to -0.2 V. 

If the reduced form of the metal ion is more stabilized by complexation than the oxidized form, then, according to the Nernst equation, the redox potential will increase. If the oxidized form is stabilized more than the reduced form, then the opposite is found. 

Stabilization by complexation depends upon a large number of interrelated factors which might depend upon the nature of the metal ion, the properties of the available ligands, or both. For example, one important factor is coordination number which, although largely determined by the electronic configuration of the metal ion, also may be controlled by the properties of the available ligands. 

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