O species

The gas temperature of the Ar plasma in the range of 600 100 K effects temperatures of water or LN2 cooled samples of 350 30 or 200 30 K, respectively. In addition, C, N, and O species are desorbed from the chamber walls and introduced samples, effecting a plasma contamination level in the 0.01-0.1% range. 

For example, pyrolysis of the previously mentioned nido-2,3-C2B4Hg gives the 3 c/o o-species shown above, whereas under the milder conditions of photolytic closure the less-stable isomer closo-, 2-C2 i is obtained. Pyrolysis of alkyl boranes at 500-600° is a related route which is particularly useful to monocarbaboranes though the yields are often low, e.g. ... 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

Though matrix-isolated gold(O) species Au(CO) and Au(CO)2 are very unstable, the gold(I) compound Au(CO)Cl has long been known (section 4.10.5). Au(CO)Br is unstable but salts of [Au(CO)2]+ of varying stability have been made [Au(CO)2](Sb2Fn) is stable to 130°C [164]. 

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

As a final example in this section, a contribution by Grubbs et al. is discussed. The chloride-free ruthenium hydride complex [RuH2(H2)2(PCy3)2] (37) is believed to react, in the presence of alkenes, to form an unidentified ruthenium(O) species which undergoes oxidative additions with dihalo compounds, e.g., 38, to give the corresponding ruthenium carbene complex 9 (Eq. 4) [20]. 

D. Tsiplakides, S. Neophytides, and C.G. Vayenas, Investigation ofthe state ofthe electrochemically generated adsorbed O species on Au films interfaced with Y203-doped-ZrOj, Ionics, submitted (2001). 

The backspillover O species on the Pt surface have an O Is binding energy 1.1 eV lower than on the same surface under open-circuit conditions. The Pt catalyst-electrode is surrounded by isoenergetic oxygen species both at the Pt/YSZ and at the Pt/vacuum interfaces.67... 

Figure 11.7 confirms that electrochemically induced and controlled O2 backspillover from the support to the metal film surface is the promoting mechanism both in the case of YSZ (Fig. 11.7a) and in Ti02 (Fig. 11.7b). These figures show the Ols spectrum of the Pt film deposited on YSZ and on TiC>2, first under open-circuit conditions (Fig. 11.7aC, 11.7bA) and then under positive current and potential application (Fig. 11.7aB, 11.7bB). Figures 11.7aC and 11.7bC show the difference spectra. In both cases, XPS clearly shows the presence of the O2 double layer, even under open-circuit conditions (Figs. 11.7aA, 11.7bA) and also clearly confirms the electrochemically controlled backspillover of O2 from the YSZ orTi02 support onto the catalyst surface. Note that the binding energy of the backspillover O species is in both cases near 529 eV, which confirms its strongly anionic (probably O2 ) state.31,32...

Virtually all work on nickel isocyanide complexes centers on nickel(O) species. Malatesta and Bonati 90) describe complexes of the formula NiL4 and Ni(CO) L4 jj. The former are formed in a variety of reactions, including reductions of nickel(II) in the presence of isocyanides, and by the replacement of other ligands by isocyanides. The latter are, of course, derivatives of Ni(CO)4. In addition, a few ill-defined nickel(II) complexes are reported, as is the formally nickel(I) species (C5H5NiCNC6Hj)2. 

The enzymatic reactions of peroxidases and oxygenases involve a two-electron oxidation of iron(III) and the formation of highly reactive [Fe O] " species with a formal oxidation state of +V. Direct (spectroscopic) evidence of the formation of a genuine iron(V) compound is elusive because of the short life times of the reactive intermediates [173, 174]. These species have been safely inferred from enzymatic considerations as the active oxidants for several oxidation reactions catalyzed by nonheme iron centers with innocent, that is, redox-inactive, ligands [175]. This conclusion is different from those known for heme peroxidases and oxygenases... 

A palladium(O) species nicely catalyzes the Grob-type decarboxylative ring-opening reaction of cyclic carbonate 67 (Eq. 17) [38,39]. 

Methanol is not active at 200°C (473 K) (there is no total oxidation by O species adsorbed at this temperature), but it appears to be a good reductant at high temperature, i.e. methanol is scavenging oxygen species left by previous NO decomposition (Figure 5.7b). 

The role of OSC materials in NO conversion is more complex. Most metals, specially Rh, are able to decompose NO, but this reaction is rapidly inhibited by O species resulting from this decomposition [60]. On Rh, for instance, Rh-0 species are replaced by Rh-NO+ ones in which NO is no longer dissociated [61]. O species may react with adsorbed species of the reducer (CO, HC) to form C02. The first role of the OSC materials could be to liberate metal sites by accepting O species. Indirect effects can also occur, with the Ce3+/Ce4+ redox system being able to regulate the metal state (zero-valent or ionic) during richness oscillations. 

The promotion by reduced ceria could be due to a spillover phenomenon of O species from metal to support. In fact, this is not sufficient to explain all the results of Mullins and Overbury. An exposure of the Rh/CeOx surface to water leads to a re-oxidation accompanied by a hydroxylation of the support while the metal surface is left unchanged. In fact, it seems that preferential orientation of Rh surface on reduced ceria may also explain the specific role of CeOx surface. This is consistent with the fact that NO dissociation occurs at lower temperatures on Rh (110) and on Rh (100) than on Rh (111) [83,84],... 

Mono- and bis(silyl)platinum(II) complexes are believed to play important catalytic roles in hydrosilylation, dehydrocoupling, and double silylation reactions with disilanes and hydrosilanes. A stable, mono(silyl)platinum(II) complex has been prepared by the oxidative addition reaction of the sterically hindered, primary arylsilane 2,6-Mes2C6H3SiH3 (Mes = 2,4,6-trimethylbenzene) to the platinum(O) species [Pt(PPr3)3] in hexane solution at room temperature.133 The colorless product m-[PLl 1(2,6-Mes2C6II3(11 )2Si)(PPr3)2] (21) was isolated as the OPPr3 adduct, and its... 

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

Palladium-catalyzed carbon-carbon cross-coupling reactions are among the best studied reactions in recent decades since their discovery [102, 127-130], These processes involve molecular Pd complexes, and also palladium salts and ligand-free approaches, where palladium(O) species act as catalytically active species [131-135]. For example, the Heck reaction with aryl iodides or bromides is promoted by a plethora of Pd(II) and Pd(0) sources [128, 130], At least in the case of ligand-free palladium sources, the involvement of soluble Pd NPs as a reservoir for catalytically active species seems very plausible [136-138], Noteworthy, it is generally accepted that the true catalyst in the reactions catalyzed by Pd(0) NPs is probably molecular zerovalent species detached from the NP surface that enter the main catalytic cycle and subsequently agglomerate as N Ps or even as bulk metal. 

Undoubtedly, the most controversial spectrum in surface studies has been that of the O- species. As indicated in Appendix C, the theoretical spectrum depends upon the energy level configuration for the 2p orbitals. For the energy level scheme shown in Fig. 20a the principal components of the g tensor are approximately given by... 

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