Noble transition metal complexes

The alkaline and rare-earth metals, and positive actinide ions, generally have greater affinity for —0 groups as electron donors. Many transition metals complex preferentially with enoHc —0 and some nitrogen functions. PolarizabiUty of the donor atoms correlates with stabiUty of complexes of the heavier transition metals and the more noble metal ions. 

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

Of course, commercially available transition metal complexes are stable at room temperature because they have achieved an 18-electron noble gas-like electronic configuration. Thus, molecules like iron pentacarbonyl [Fe(CO)s], ferrocene [Fe(C5H5)2], as well as piano-stool complexes such as C5H5Co(CO)2 are chemically quite inert. In order to study bimolecular reactions, it is necessary to first prepare unsaturated complexes. For studies using molecular beams, one approach is through photolysis of a stable volatile precursor in a supersonic nozzle. 

The interaction of saturated C—H and C—C bonds with heterogeneous metal catalysts forms the basis of widely applied reactions such as isomerization, cracking, and re-forming of alkanes. In recent years, much attention has been devoted to the selective activation of C—H bonds by transition metal complexes in homogeneous solution under mild conditions.601 604 In principle, an alkane can undergo oxidative addition to a noble metal complex according to... 

Electron counting in transition metal complexes (18 e rule) is an useful tool to understand their stability and structure, although it does not apply to all transition metal complexes—only for a majority of compounds containing 7i-acceptor ligands. The 18 electron rule is an extension of the idea of the octet rule, which applies to atoms having only s and p orbitals. The idea is that the molecule will be stable when the central atom has the same electronic structure as noble gases of the same row. A similar concept can be applied to transition metal complexes having d electrons. The compound is considered most stable when the total number of electrons around the atom becomes the... 

Since Pt dissolution is favored by high electrode potential, relative humidity, and temperature, the possibility to limit the risk of electrocatalyst aging is based on the use of Pt-alloy catalyst instead of pure platinum, at least for the cathode, which is characterized by higher potential with respect to anode, and by adoption of operative conditions not too severe in terms of humidity and temperature. While this last point requires interventions on the membrane structure, the study of catalyst materials has evidenced that a minor tendency to sintering can be obtained by the addition of non-noble metals, such as Ni, Cr, or Co, to the Pt cathode catalyst [59, 60], suggesting a possible pathway for future work. On the other hand also the potential application of non-platinum catalysts is under study, in particular transition metal complexes with structures based on porphyrines and related derivatives have been proposed to substitute noble metals [61], but their activity performance is still far from those of Pt-based catalysts. 

The influence of closed-shell transition-metal complexes may be drastically influenced by the cations surrounding the complex. Up till now we have neglected this influence. This is correct if these cations have their excited states at energies far above the emitting level of the complex. This is, for example, the case for noble-gas ions. 

This is a well-established class of reaction and several reviews have been published 2-3). The reaction is widely observed with transition metal complexes, especially with noble metal complexes, and is most important for the direct formation of a-bonds from metal complexes and simple molecules. The reaction can be expressed by the following scheme ... 

Homogeneous transition metal catalyzed reactions have been very well studied over the past 50 years. The metal complexes used to accompUsh these reactions have typically included noble transition metals or first row metals. Due to the cost and pathogenicity of these noble metals, the development of new catalytic routes using non-precious metals is an active area of research. In this chapter, a review of the use of selected cobalt and nickel species towards catalytic reactions analogous to those seen with noble metals is presented. 

Over the last decade, the copper-mediated or copper-catalyzed C-H functionalization has been developed rapidly and greatly by significant efforts of many researchers, and cheap and abundant copper salts now can replace, to some extent, precedented noble transition metal catalysts such as Pd, Rh, and Ru. Moreover, some unique features of copper salts and complexes are observed. The intermolecular dehydrogenative cross-couplings mentioned in this chapter are such good examples, and they are otherwise challenging even under known noble transition metal catalysis. However, there is still a large room for further... 

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