Redox Chemistry of the Transition Metals

The transition metals, unlike those in Groups 1 and 2, typically show several different oxidation numbers in their compounds. This tends to make their redox chemistry more complex (and more colorful). Only in the lower oxidation states (+1, +2, +3) are the transition metals present as cations (e.g., Ag+, Zn +, Fe ). In higher oxidation states ( + 4 to +7) a transition metal is covalently bonded to a nonmetal atom, most often oxygen. 

We should point out that many oxides of the transition metals beyond those listed in Table 20.2 can be prepared indirectly. For example, although silver does not react directly with oxygen, silver(I) oxide, Ag20, can be made by treating a solution of a silver salt with strong base. 

In another case, cobalt(II) oxide can be prepared by heating the carbonate in the absence of air. 

Any metal with a positive standard oxidation voltage, x can be oxidized by the ions 

Nickel reacts slowly with hydrochloric acid to form H2(g) and N 2+ ions in solution. Evaporation of the solution formed gives green crystals of NiCh-eHsO. 

Products of Reactions of the Transition Metals with Oxygen 

Many oxides of the transition metals can be nonstoichiometric the atom ratios of the elements differ slightly from whole-number values. Stoichiometric nickel(II) oxide, in which N 2 O2- Ni2 o2- 

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Section 20.1 deals with the processes by which these metals are obtained from their principal ores. Section 20.2 describes the reactions of the alkali and alkaline earth metals, particularly those with hydrogen, oxygen, and water. Section 20.3 considers the redox chemistry of the transition metals, their cations (e.g., Fe2+, Fe3+), and their oxoanions (e.g., Cr042-). ... 

The enhanced ORR activities of the dealloyed Pt binary catalysts could be related to a similar lattice-strain effect as revealed in the dealloyed PtCus catalysts. However, it is still unclear what the origin of the different activities of different dealloyed PtM3 catalysts is. Regarding their comparable atomic radius, the alloy elements Co, Ni, and Cu are assumed to induce a similar extent of lattice strain. Nevertheless, due to different redox chemistry of the transition metals, different extent of metal dissolution may exist [47], which might result in different core-shell fine structures and hence different activities. 

The chemistry of the transition metals including chromium(III) with these ligands has been the subject of a recent and extensive review,788 with references to the early literature. The close relationship between the catechol (180), semiquinone (181) and quinone (182) complexes may be appreciated by considering the redox equation below (equation 44). 789 The formal reduction potentials for the chromium(III) complexes (183-186 equation 45) are +0.03, -0.47 and -0.89 V (vs. SCE in acetonitrile) respectively. 

Iron complex photolysis is mie of the processes that produce reduced iron (Fe(n)) in a highly oxidizing enviromnent like the atmospheric aqueous phase. There are numerous other processes such as reactions with HO species or Cu(I)/ Cu(n) which can reduce or oxidize iron in the troposphere. These reactions can take place simultaneously and cause iron to undergo a so-caUed redox-cycling [167]. Because of the large number of complex interactions in the atmospheric chemistry of the transition metal iron, it is useful to utilize models to assess the impact of the complex iron photochemistry. 

A particular feature of transition metal porphyrin chemistry is that the energies of the metal d orbitals and the frontier orbitals of the porphyrin ligand arc often quite close, with the result that the redox chemistry of the porphyrin ligand and the... 

In the transition metal complexes of ligands like o-phenylenediamine, we find a distinct analogy to the chemistry of dithiolenes. The full equivalence of their redox chemistry with that of dithiolenes suggests a comparably delocalized structure. The outward structural similarity between this ligand and the benzenedithiols is not reflected in the redox properties of their transition metal complexes... 

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. 

A range of chemical analogs of the catalytic centers of Mo and W dithiolene-containing enzymes (pterins) have been prepared. In particular, the rich chemistry of multisulfur transition metal systems allows ligand redox, internal electron transfer, and intermediate redox states. Such redox flexibility may facihtate coupled proton/electron transfer and/or 0x0-transfer mechanisms, which are employed by Mo and W enzymes. 

From the examples above, it is clear that oxidation reactions using redox-active molecular sieves is an active area of research for the synthesis of fine chemicals and intermediates. The novel chemistry is possible because of the location of the transition metal ions at specific crystallographic sites in the framework and also the pore structure of microporous materials. 

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. 

A proper description of electronic defects in terms of simple point defect chemistry is even more complicated as the d electrons of the transition metals and their compounds are intermediate between localized and delocalized behaviour. Recent analysis of the redox thermodynamics of Lao.8Sro,2Co03. based upon data from coulometric titration measurements supports itinerant behaviour of the electronic charge carriers in this compound [172]. The analysis was based on the partial molar enthalpy and entropy of the oxygen incorporation reaction, which can be evaluated from changes in emf with temperature at different oxygen (non-)stoichiometries. The experimental value of the partial molar entropy (free formation entropy) of oxygen incorporation, Asq, could be... 

The template removal step, needed to achieve porous materials, is one of the most critical points. In contrast to silica, other compositions are usually more sensitive to thermal treatments and calcination can result in breakdown of the mesostructures. Hydrolysis, redox reactions, or phase transfonnarions to the thermodynamically preferred denser crystalline phases account for this lower thermal stability. Many of the transition metal-based mesostruetured materials synthesized in the presence of cationic surfactants collapse during thermal treatments. The poor thermal stability observed could be due to the different 0x0 chemistry of the metals compared to silicon. Several oxidation states of the metal centers may be responsible for oxidation and/or reduction during calcination. In addition, incomplete condensation of the framewoik is possible. 

Reactivity modes of the poly(pyrazolyl)borate alkylidyne complexes follow a number of recognised routes for transition metal complexes containing metal-carbon triple bonds, including ligand substitution or redox reactions at the transition metal centre, insertion of a molecule into the metal-carbon triple bond, and electrophilic or nucleophilic attack at the alkylidyne carbon, C. Cationic alkylidyne complexes generally react with nucleophiles at the alkylidyne carbon, whereas neutral alkylidyne complexes can react at either the metal centre or the alkylidyne carbon. Substantive work has been devoted to neutral and cationic alkylidyne complexes bearing heteroatom substituents. Differences between the chemistry of the various Tp complexes have previously been rationalised largely on the basis of steric effects. 

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