Metal participation

Autoxidation. In this category, which refers to direct reactions of substrates with oxygen, there are numerous examples of metal catalysis of hydrocarbon oxidation. While metal participation most frequently involves catalysis of hydroperoxide decomposition (21, 22) (Reactions 12 and 13),... 

The arrow in 11a symbolizes donation of tt electrons. However, because the stability of the ion is much greater than would be expected for either a simple acid-base or charge-transfer complex, it is postulated that unshared d electrons from the metal participate in the bonding. This is symbolized by the dashed arrow in 11b, which stands for donation of d electrons into the tt orbital of the double bond or, as it is often called, back bonding. Perhaps most simple is 11c, where the C-Pt bonding is formulated as a three-... 

As seen from this review, CO2 diemistry is likely to have its second rebirth when it is extended to systems with transition metal participation. Even the first results disclose numerous possibilities for using CO2 in different chemical processes in the future. The industrial chemist is especially interested in producing usable compounds which are formed from the cheap chemical CO2. Further research should be done on the hydrocarboxylatton reactions of alkenes whereby saturated or unsalurated carboxylic acids are obtained. Aromatics or alkanes with activated hydrogen should also be possible reaction partners of carb(Hi dioxide. Future work will extend our knowledge of the scope of transition metal-C02 chemistry and of the potential uiUity of carbon dioxide as a feedstock for the production of organic chemicals. 

It is aimed, in the present investigation, to focus on photophysical properties, which depend on the metal character and which can be tuned by changing the metal participation. The increase of metal character is particularly distinct when Pd(II) complexes are compared to the corresponding Pt(ll) complexes. For example, the zero-field splitting of the lowest triplet - reflecting the metal character - increases by nearly a factor of a hundred, when Pd(2-thpy)2 is compared to Pt(2-thpy)2 (Fig. 1). This is due to the significantly greater MLCT (dn ) admixtures to the lowest LC (titt ) states in the Pt(II) complex as compared to the Pd(II) complex. 

Fig.1. The compounds are arranged according to an increasing total zero-field splitting (zfs) of the lowest excited triplet. This splitting reflects the size of metal participation (MLCT and/or d-orbital character) in the corresponding wavefunctions...

It is doubtful that orbital symmetry conservation will maintain pronounced constrictive control over cycloaddition processes when transition metals participate. Certainly, the absolute order reflected in the sharp division between allowed and forbidden processes found in organic chemistry would not be anticipated in this special area. The reasons are primarily those just discussed. But the importance of orbital symmetry control does not rest on its predictive powers alone. It provides insight into the nature of a molecular transformation, focusing attention on the character of transforming bonds as reflected in the symmetries of their composite molec-... 

The problem of metal participation in the properties of a-metallo-cenyl-carbonium ions Determination of absolute configurations of axial- and planar-symmetrical compounds Ferrocene-type complex compounds and organic synthesis Chemistry of ferrocenes... 

Large line-widths, sometimes in conjunction with insufficiently resolved hyperfine structure, can preclude the determination of individual g factor components close to g = 2 under conventional ESR conditions (X band, 9.5 GHz, 330 mT). As in NMR spectroscopy, an increase in the field and frequency is then appropriate via the Q band (35 GHz) to the W band (95 GHz) or even higher frequencies and magnetic fields [48]. An application of such high-field ESR for the inorganic radical 10 with sizeable metal participation has been reported [49a]. 

This catalyst [111] gives several polymer molecules per titanium atom, and there are similar findings for Cr(acac)3/AlEt3 for butadiene and Cr(acac)3/AlEt2 Cl or AlEtCl2 for ethylene [139]. It appears reasonable to assume, therefore, that all the transition metal participates in the reaction. With Cr(acac)3/AlEt2Cl the average number of polymer molecules approximates to one per Cr atom which additionally would indicate the absence of a transfer reaction. This point needs to be confirmed, however, with separate measurements of active centre concentrations and chain transfer, since two soluble nickel catalysts [68, 124] have been reported to have low catalyst efficiencies. 

Catalytic experience tells us that frequently only a small fraction of the sites at the surface of the metal participate in the reaction. Recently, Krai measured the metal surface area of palladium catalysts supported on carbon, their specific activity for various hydrogenation reactions, and their poisoning by thiophene (44). By combining the classic technique of poisoning with the measurement of metal surface area. Krai was able to show that the Taylor ratio, i.e., the fraction of active sites in each particular reaction, changed from unity to about 10 . With a Taylor ratio of unity, the reaction would be called facile. Otherwise, we deal with a structure-sensitive or demanding reaction. 

The electronic interaction resulting in the observed stability and stereospecificity is still a matter of controversy. One proposal is direct metal participation, involving a... 

Metal cofactors in enzymes may be bound reversibly or firmly. Reversible binding occurs in metal-activated enzymes (e.g., many phosphotransferases) firm (or tight) binding occurs in metalloenzymes (e.g., carboxypeptidase A). Metals participate in enzyme catalysis in a number of different ways. An inherent catalytic property of a metal ion may be augmented by the enzyme protein, or metal ions may form complexes with the substrate and the active center of the enzyme and promote catalysis, or metal ions may function in electron transport reactions between substrates and enzymes. 

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