Transition elements, redox reactions involving

Although several research groups have been interested in transition metal enolates to use the metal centre as a potential site of asymmetry in the design of chiral catalysts, examples of well defined redox reaction involving middle to late transition elements and lanthanides are scarce in the literatnre. Based on Pearson s theory of hard and soft acids and bases", it has been proposed that combining a hard ligand with a soft late transition metal centre may lead to weak metal-heteroatom links, resnlting in reactive late metal-heteroatom bonds. 

Only simple outer-sphere (25) redox reactions involving, for example, complex or aquo ions of transition or certain rare earth elements do not experience electrocatalysis, and their standard rate constants are independent of electrode material. This is because neither the oxidized nor the reduced species are chemisorbed at the electrode. However, practically, many redox systems do experience electrocatalysis on account of significant adsorption of their ions or through mediation of electron transfer by adsorbed anions, in which case the processes are no longer strictly of the outer-sphere type. 

One can then attempt to relate the free energy of the reaction (or the equilibrium constant or the redox potential) of the one-electron step to the rate of the reaction. Extensive tabulations of one-electron redox potentials have recently become available (e.g., Wardman, 1989). Often it is possible to relate rate constants to free energy parameters (AG , K, pe°) in a series of related redox reactions (e.g., oxidation of ions of transition elements with O2, H2O2, Mn02, etc.) or redox reactions involving organic compounds with var-... 

Oxidation state-potential diagrams for nonmetallic and transition metal elements provide an interesting framework for analyzing the highly varied results obtained for redox reactions involving as many as nine oxidation states. So many different products and stoichiometries are obtained from the reduction of nitric acid that early work seeking patterns of reaction was abandoned after many years of frustrating effort. 

Many of the reactions involving transition metals are redox reactions (Chapter 9) involving electron transfer. These can be described by half-equations and by ionic equations formed from the combination of two half-equations (with equal numbers of electrons). Disproportionation is a redox reaction involving the simultaneous increase and decrease in oxidation state of the same element. 

The possible states of electrons are called orbitals. These are indicated by what is known as the principal quantum number and by a letter—s, p, or d. The orbitals are filled one by one as the number of electrons increases. Each orbital can hold a maximum of two electrons, which must have oppositely directed spins. Fig. A shows the distribution of the electrons among the orbitals for each of the elements. For example, the six electrons of carbon (B1) occupy the Is orbital, the 2s orbital, and two 2p orbitals. A filled Is orbital has the same electron configuration as the noble gas helium (He). This region of the electron shell of carbon is therefore abbreviated as He in Fig. A. Below this, the numbers of electrons in each of the other filled orbitals (2s and 2p in the case of carbon) are shown on the right margin. For example, the electron shell of chlorine (B2) consists of that of neon (Ne) and seven additional electrons in 3s and 3p orbitals. In iron (B3), a transition metal of the first series, electrons occupy the 4s orbital even though the 3d orbitals are still partly empty. Many reactions of the transition metals involve empty d orbitals—e.g., redox reactions or the formation of complexes with bases. 

It has been already emphasized that substitution of heteroelements into the framework of molecular sieves creates acidic sites. Incorporation of transition elements such as Ti, V, Mn, Fe, or Co, which have redox properties, provides molecular sieves with redox active sites that are involved in oxidation reactions (323-332). As mentioned in the beginning of the article, the transition metal-substituted molecular sieves, the so-called redox molecular sieves, exhibit several advantages compared with other types of heterogeneous redox catalysts (1) redox sites are isolated in a well-defined internal structure therefore, oligomerization of the active oxometal species is prevented (this is a major reason for the deactivation of homogeneous catalysts) (2) the site isolation (the so-called microenvironment) of redox centers prevents the leaching of the metal ions, which frequently happens in liquid-phase oxidations catalyzed by conventional transition metal-supported catalysts (3) well-defined cavities and channels of molecular dimensions endow the catalysts with unique performances such as the shape selectivity (and traffic control) toward reactants, intermediates, and/or products. 

After being demonstrated for the first time for transition metal oxides [POI 00], the conversion reaction (equation [1.3]) has since been expanded to a number of other elements (X = O, S, P, F, Sb...). This profound transformation of the initial material MaXb into a composite electrode made up of metallic nanoparticles and a Li X matrix enables high energy densities to be reached. Moreover, it involves redox reactions very different from those of the insertion mechanisms, which only involve the transition metal, whereas here the transition metal and post-transitional element are simultaneously reduced or oxidized. These conversion reactions thus enable more than 1 Li (le ) to be exchanged per metallic atom, and result in gravimetric and volumetric capacities that can reach 1,000 mAh/g, and 7,000 mAh/cm, respectively, which is nearly 10 times that of graphitic carbon (800 mAh/cm ). Until recently, these materials were only a laboratory curiosity since the conversion reaction, although reversible, did... 

Many reactions involving transition elements are redox reactions. Some redox reactions are used in titrations to determine concentrations. 

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