Redox potentials of transition metal

Redox potentials of transition metal ions in aqueous solutions... 

Under appropriate conditions, analysis of cychc voltanuno-grams such as that shown in Figure 1 provide simple measures of the solution redox potentials of transition metal complexes, derived as 1/2. Extensive tabulations of redox potentials are available, and provide a key resource for inorganic chemistry. The correlation redox potentials with parameters such as ligand field strength, ligand hardness, and ionization energy of the transition metal forms an important theoretical framework for transition metal chemistry. 

Several approaches have been made (see Reference 96 and references cited therein) to establish a quantitative, empirical scale describing the influence of a large collection of ligands of wide structural diversity on the redox potentials of transition metal complexes. All the approaches are based on the assumption that the relative contribution of each ligand to the redox potential of a transition metal complex is independent of the presenee of other ligands, the identity of the transition metal, the oxidation and spin states of this metal, the solvent etc. . Hence the approaches resemble the empirical use of substituent constants in the Hammett relation in organic chemistry. 

TABLE 18. Examples of the influence of M (P, As, Sb) in ligands on the redox potentials of transition metal complexes"... 

A relatively large number of theoretical studies on the complete calculation of redox potentials of transition metal active sites in metalloproteins have been published. Metalloproteins studied include manganese superoxide dismutase, iron superoxide dismutase, copper-zinc superoxide dismutase, iron-sulfur proteins, cytochrome f, components of the photosynthetic reaction center, and peroxidases. ... 

Table 3.3 Redox potentials of transition-metal sandwich complexes obtained by cyclic voltammetry in different ionic liquids...

One-electron reduction or oxidation of organic compounds provides a useful method for the generation of anion radicals or cation radicals, respectively. These methods are used as key processes in radical reactions. Redox properties of transition metals can be utilized for the efficient one-electron reduction or oxidation (Scheme 1). In particular, the redox function of early transition metals including titanium, vanadium, and manganese has been of synthetic potential from this point of view [1-8]. The synthetic limitation exists in the use of a stoichiometric or excess amount of metallic reductants or oxidants to complete the reaction. Generally, the construction of a catalytic redox cycle for one-electron reduction is difficult to achieve. A catalytic system should be constructed to avoid the use of such amounts of expensive and/or toxic metallic reagents. 

Nanoparticles of light transition metals are more difficult to prepare because of lower redox potential of the metal ions than those of the precious metal ions. In other words, light transition metal ions are difficult to reduce to zero valence, and zero-valence light transition metals are easily oxidized to ions. 

Transition metals in a high oxidation state are often capable of extracting an electron from electron-rich organic substances. Ketones, esters, nitriles and various other carbon acids that can form enols, enolates and related structures are by far the most commonly used substrates. Their oxidation can lead to a free radical, which then follows one or more of the pathways deployed in Scheme 8.3. Its important to take into account that the rate of radical production will depend on the exact structure of the substrate, its propensity to exist as the corresponding enol or enolate in the medium, the pH, the solvent, the temperature and, of course, the redox potential of the metallic salt (which can be strongly affected by the nature of the ligand around the metal) and the exact mechanism by which electron transfer actually occurs (i.e. inner or outer sphere)... 

The increase of the redox potential of a metal cluster in a solvent with its nuclearity is now well established 1-4). The difference between the single atom and the bulk metal potentials is large (more than 2 V, for example, in the case of silver (3)). The size dependence of the redox potential for metal clusters of intermediate nuclearity plays an important role in numerous processes, particularly electron transfer catalysis. Although some values are available for silver clusters (5, 6), the transition of the properties from clusters (mesoscopic phase) to bulk metal (macroscopic phase) is unknown except for the gas phase (7-9). 

Zhou F, Cococcioni M, Marianetti CA, Morgan D, Ceder G (2004) First-principles prediction of redox potentials in transition-metal compounds with LDA -i- U. Phys Rev B 70 235121 1-8... 

The effects of concentration, velocity and temperature are complex and it will become evident that these factors can frequently outweigh the thermodynamic and kinetic considerations detailed in Section 1.4. Thus it has been demonstrated in Chapter 1 that an increase in hydrogen ion concentration will raise the redox potential of the aqueous solution with a consequent increase in rate. On the other hand, an increase in the rate of the cathodic process may cause a decrease in rate when the metal shows an active/passive transition. However, in complex environmental situations these considerations do not always apply, particularly when the metals are subjected to certain conditions of high velocity and temperature. 

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

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