TOPICAL redox change

To continue the present topic, we would like to underline that often irreversible redox processes do not cause the appearance of new, well-defined redox waves such as to induce an accurate examination of the underlying chemical complications. Nevertheless, not always irreversible redox changes lead to complete destruction of the metal complex under examination. Analysis of the products arising from irreversible processes can sometimes reveal interesting chemical pathways. 

Finishing this chapter, we should note that such an interesting field as the EPR spectroscopy in studies of photochemical, photoelectrochemical and photocatalytic processes has not been reviewed. Indeed, doping oxide semicon-ductors with Fe3+, Mo5+, Cu2+, Cr3+, Nb5+, V4+ Mn2+ or Rh3+, etc., one can await changes in the photoreactivity and changes of the valency of these ions involved in the redox chemical processes, which can be easily detected and interpreted, providing valuable information on the studied systems. Unfortunately, the limited space of this book, does not allow us to discuss this topic, which will be done elsewhere. 

Rate acceleration is the most fimdamental aspect of catalysis. To elucidate the electronic mechanisms of spin-acceleration phenomena therefore represents an important topic in both bioinorganic and biomimetic dioxygen activation 5,122). Many other t5rpes of substrate transformations catalyzed by metal complexes or redox enzymes also involve key steps with a change in spin along their reaction coordinates. The abimdance of such phenomena seems to be much wider than initially thought. This... 

Kochi s book from 1978 [la] helped to establish electron-transfer and radical reactions as a crucial part of mainstream organometallic chemistry. The importance of such reactions is evident from Astruc s book [lb], still the most comprehensive and authoritative book in the area, and from several reviews and review collections [2] on aspects of organometallic electron-transfer reactivity. This chapter will be fully devoted to the use of electrochemical techniques to obtain bond-energy data for organometallic complexes, a topic that has not been previously reviewed. Aspects of the energetics of redox-induced structural changes and isomerizations, a thoroughly pursued topic, has been reviewed [2o] and will not be included here. 

In an electrochemical cell a redox reaction occurs in two halves (see Topic B4). Electrons are liberated by the oxidation half reaction at one electrode and pass through an electrical circuit to another electrode where they are used for the reduction. The cell potential E is the potential difference between the two electrodes required to balance the thermodynamic tendency for reaction, so that the cell is in equilibrium and no electrical current flows. E is related to the molar Gibbs free energy change in the overall reaction (see Topic B3) according to... 

A comprehensive review of o-bonded iron porphyrin electrochemistry has recently been published [12], and results on these compounds will not be discussed in the present paper. Several reviews have been published on the redox tuning of iron porphyrins over the last 20 years [2, 7, 10, 192] and this topic will also not be covered in the present paper. The exact potential for the Fe(III)/Fe(II) reaction will depend on the type of axial ligand coordinated to the Fe(III) or Fe(II) forms of the porphyrin. Axial ligands such as NO, C6H5 and 0104 will change drastically the potential at which the Fe(III)/Fe(II) redox couple is observed, but shifts of E /i for this redox reaction will occur upon solvent binding to the Fe(III) and/or Fe(II) form of the compound. The basicity of the porphyrin macrocycle will also influence E ji for the Fe(II)/Fe(III) electrode process. 

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