Effects on Redox Properties

Finally, the electrochemical data for the cobalt system enable evaluation of the effect on redox properties of the thiometalate ligands (TM) varying from M02S2- to MS2-. It is evident that an-increasing oxygen content of the ligand makes the reduction more difficult. 

NEW POLYDENTATE LIGANDS AND COMLEXES PROTIC AND TOPOGRAPHIC EFFECTS ON REDOX PROPERTIES... 

New Polydentate Ligands and Complexes Protic and Topographic Effects on Redox Properties M. Gross, A. Nurhadi and E. Grttf... 

The consequences of polychlorination of porphyrins on redox properties of complexes has been investigated.1404 The highly chlorinated porphyrin 3-octachloro-/ /c.vo-tetrakis(3,5-dichloro-2,6-dimethoxyphenyl)porphyrin exhibits a substantial anodic shift for reduction of over 0.5 V and a smaller shift for oxidation versus the unchlorinated precursor. Contrastingly, small potential shifts for the octabromo-substituted 5,10,15,20-tetraphenylporphyrinate arise from the dominance of macrocycle ruffling over electronic effects. In the polychloro complex, distortion does not compensate fully for electron-withdrawing effects of the Cl substituents. 

Isotope effects on redox reactions of the type considered in Section 2.6 are of interest for a number of reasons. At a fundamental level, the magnitude of the effect provides an important clue to the electronic structure and vibrational properties of the species involved. From a practical point of view, a large deviation from unity for the equilibrium constant offers a convenient procedure for the enrichment of the isotopomer mixtures. 

This chapter deals with the fundamental aspects of redox reactions in non-aque-ous solutions. In Section 4.1, we discuss solvent effects on the potentials of various types of redox couples and on reaction mechanisms. Solvent effects on redox potentials are important in connection with the electrochemical studies of such basic problems as ion solvation and electronic properties of chemical species. We then consider solvent effects on reaction kinetics, paying attention to the role of dynamical solvent properties in electron transfer processes. In Section 4.2, we deal with the potential windows in various solvents, in order to show the advantages of non-aqueous solvents as media for redox reactions. In Section 4.3, we describe some examples of practical redox titrations in non-aqueous solvents. Because many of the redox reactions are realized as electrode reactions, the subjects covered in this chapter will also appear in Part II in connection with electrochemical measurements. 

An approach to quantifying the interaction between solute and solvent and hence to solvent effects on redox potentials is that developed by Gutmann.41 Interactions between solvent and solute are treated as donor-acceptor interactions, with each solvent being characterized by two independent parameters which attempt to quantify the electron pair donor properties (donor number)... 

C. Effects of Constituent Elements on Redox Properties in the Solid State... 

The effect of complexation on redox properties was studied by cyclic voltammetry. Unbound flavin, dissolved in an aprotic solvent (dichloromethane), undergoes a two electron reduction perfectly explained by the ECE mechanism. Upon addition of cyclene ligand and coordination of flavin to the zinc ion complex, the flavohydroquinone redox state was stabilised. 

In contrast to the HOMO and LUMO, the singly occupied molecular orbital (SOMO) can be correlated both qualitatively and quantitatively with experimentally measurable EPR hyperfine couplings (hfcs). As a result, host-guest systems that have redox-active guests that are stable as radicals provide excellent tools for studying the effects of noncovalent interactions on redox properties. 

Most studies indicate that the metal has the greatest effect on the properties of the polymer when it is part of the backbone, rather than a sidegroup. The direct coupling of the metal orbitals with the Tr-orbitals of the conjugated polymer may allow redox-matching to enhance the conductivity and perturb the band structure of the polymer. However, it is often synthetically easier to add the metal center as a side chain of a known polymer (e.g., on PPV) than to develop new polymerization procedures for the metal complex. 

Electrochemical, magnetic, and spectroscopic properties are reported for divalent and trivalent homoleptic [Fc(Tp)2 +//°, [Fe(Tp )2]+/°, and [Fe(pzTp)2]+7°.272 Ligand field strengths for these metal complexes increase as Tp < Tp < pzTp, which reflects the importance of steric rather than electronic effects on spectroscopic properties. Metal-centered redox potentials become more negative as pzTp < Tp < Tp, which follows the electron-donating ability of the ligands.47... 

Reviews of interest include a general review of anation reactions of cobalt(III) complexes and a discussion of the solvation of transition metal complexes/ The nature of the solvent can have a very large effect on such properties as solubilities, reactivities, redox potentials, formation constants, and various types of spectra. Such solvent effects reflect changes in the solvation of ions, complexes, initial states, transition states, and excited states. 

In this review, after a brief overview of the structural and electronic properties of metal adlayers, there are six sections describing catalytic effects on redox couples, oxidation of organic molecules, carbon monoxide, organic electrosynthesis reactions, hydrogen evolution, oxygen reduction, and metal electrodeposition. Outside the scope of this review are other UPD processes that play a role in determining the catalytic properties of electrode surfaces such as the UPD of H and OH. 

This empirical equation (when corrected for the effects of inter-electronic repulsion and for differences in the ligand field splitting) allows one to predict the frequencies of charge transfer bands for a great variety of complexes . Remarkable information on redox properties can be obtained from the parameters of the Jorgensen equation because they are roughly proportional to valence state ionization potentials of the metal and to the ionization potentials of the ligands. 

ELECTROCHEMISTRY OF FISCHER AMINOCARBENE COMPLEXES EFFECTS OF STRUCTURE ON REDOX PROPERTIES, ELECTRON DISTRIBUTION, AND REACTION MECHANISMS... 

Wright, C.S., Walton, R.I., Thompsett, D., Fisher, J. and Ashbrook, S.E. 2007) One-step hydrothermal synthesis of nanocrystaHine ceria-zirconia mixed oxides the beneficial effect of sodium inclusion on redox properties. Advanced Materials, 19, 4500-4. 

In this work, well-defined complexes of biologically important 3d transition metals (Cu(II), Fe(III), Fe(II), and Ni(II)) with either neutral or monodeprotonated anionic adenine or adenosine, synthesized and characterized 5 as described previously, have been used as a model system to study the effects of the interaction of transition metals with purine and purine nucleoside components of nucleic acids on redox properties of the system. The structures of the complexes is simpler than that of nucleic acids and facilitates evaluation of the electrochemical results. The non-phosphorylated monomeric units are suitable model ligands as the use of nucleotides offers complicating factors associated with phosphate due to self-association and self-complexation and preference for the PO4 moiety as the site for complexation. ... 

When the second-site revertants were segregated from the original mutations, the bci complexes carrying a single mutation in the linker region of the Rieske protein had steady-state activities of 70-100% of wild-type levels and cytochrome b reduction rates that were approximately half that of the wild type. In all these mutants, the redox potential of the Rieske cluster was increased by about 70 mV compared to the wild type (51). Since the mutations are in residues that are in the flexible linker, at least 27 A away from the cluster, it is extremely unlikely that any of the mutations would have a direct effect on the redox potential of the cluster that would be observed in the water-soluble fragments. However, the mutations in the flexible linker will affect the mobility of the Rieske protein. Therefore, the effect of the mutations described is due to the interaction between the positional state of the Rieske protein and its electrochemical properties (i.e., the redox potential of the cluster). 

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