Oxidative Addition Other Elements

Molybdenum.— An oxidative-elimination reaction takes place when the complexes [Mo(CO)4(5-X-phen)] (X = H, Me, Cl, or NO2) are treated with mercuric chloride in acetone. The products are [MoCl(HgCl)(CO)3(5-X-phen)). Kinetic studies, using stopped-flow techniques, indicate that the reactions proceed via initial formation of adducts of stoicheiometry [Mo(CO)4(5-X-phen)],-HgCla and [Mo(CO)4(5-X-phen)],2HgCla. These adducts are of the metal donor-metal acceptor type which were first characterized for some iron(0) and cobalt(i) complexes. - Consistent with this formulation, the rate of initial attack of HgCla on the molybdenum complexes increases as the substituent X becomes more electron-releasing. Donor-acceptor complex formation is also apparent in the reactions of iodine with the compounds [FeCCOsP and [R(acac)a].  

Chromium.—Photochemical reactions of, for example, Cr(C He)(CO)3 with silanes in hydrocarbon solvents give the silyl-transition-metal hydrides (18). The mechanism is thought to be dissociative, with loss of one carbon 

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Many of the reactions of halogens can be considered as either oxidation or displacement reactions the redox potentials (Table 11.2) give a clear indication of their relative oxidising power in aqueous solution. Fluorine, chlorine and bromine have the ability to displace hydrogen from hydrocarbons, but in addition each halogen is able to displace other elements which are less electronegative than itself. Thus fluorine can displace all the other halogens from both ionic and covalent compounds, for example... 

Only about 10 elements, ie, Cr, Ni, Zn, Sn, In, Ag, Cd, Au, Pb, and Rh, are commercially deposited from aqueous solutions, though alloy deposition such as Cu—Zn (brass), Cu—Sn (bronze), Pb—Sn (solder), Au—Co, Sn—Ni, and Ni—Fe (permalloy) raise this number somewhat. In addition, 10—15 other elements are electrodeposited ia small-scale specialty appHcations. Typically, electrodeposited materials are crystalline, but amorphous metal alloys may also be deposited. One such amorphous alloy is Ni—Cr—P. In some cases, chemical compounds can be electrodeposited at the cathode. For example, black chrome and black molybdenum electrodeposits, both metal oxide particles ia a metallic matrix, are used for decorative purposes and as selective solar thermal absorbers (19). 

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

Filter samples can be prepared to airborne workplace concentrations by spiking each filter with aqueous solution containing elements with concentrations gravimetrically traceable to ultrapure metals or stoidiiometricaUy well defined oxides. The amormts correspond for some of the materials to current threshold limit values of contaminants in workroom atmospheres provided that the simulated filter has been exposed to one cubic meter of air. The certified values are based on a gravimetric procedure, i.e. weight per volume composition of the primary reference material dissolved in high purity sub-dis-tiUed acids. The National Institute of Occupational Health in Oslo, Norway, has produced several batches of such materials certified for 20 elements. Additionally, information values are reported for four other elements see Table 6.2. 

Oxidative addition reactions have been observed also for the binuclear clusters of other d-transition elements with multiple M-M bonds [1,116,117]. The multiplicity of the M-M bonds must decrease in these reactions. It is known from organic chemistry that similar reactions are extremely characteristic of unsaturated organic compounds. We believe that the capacity for oxidative... 

Syntheses of aryl organometallics other than polyhalogenoaryls by thermal decarboxylation are comparatively rare. There are several reasons for this. For transition elements, the thermal stability of simple aryls is often low, especially by comparison with polyhalogenoaryl derivatives, thereby excluding syntheses at elevated temperatures. Electron-withdrawing substituents frequently aid thermal decarboxylation (Section III,A-D), and their absence inhibits major mechanistic paths to both transition metal and main group element derivatives, e.g., SEi (carbanionic) and oxidative addition (Section II). In thermal decomposition of... 

In the second instance, two approaches seem to be worthy of special note. The synthetic utility of elemental phosphorus based on it acting as a radical trap appears to be quite valuable, but additional effort is required to determine the variability of the source of the organic free radicals. (Is there some other, more efficacious, source of organic free radicals that works better with this system than acylated iV-hydroxy-2-pyridones ) The other approach that appears ripe for development is the hydrolysis/elimination with "phosphorates" derived from the oxidative addition of white phosphorus to alkenes. We look forward to the continued development of such facile approaches toward the preparation of fundamental phosphonic acids. 

HCHO and PH3 proceeds in the presence of K2PtCl4 at room temperature and affords the crystalline product in an essentially quantitative yield in 2.5 h [4]. Palladium compounds are also active in the catalysis [5]. In these reactions the active species is believed to be zero valent. Two mechanistic possibilities have been proposed as illustrated in Scheme 2. The first elemental process involved in the catalytic cycle is oxidative addition of a P-H bond, which is well precedented [6]. In one of the mechanistic possibilities the processes that follow the oxidative addition are the insertion of the C=0 bond into H-M species and P-C reductive elimination, the latter of which is also precedented [7]. In the other, the coordinating phosphide ligand makes a nucleophilic attack [8] at the formaldehyde carbon forming zwitterionic species. 

Although all of the above elements catalyze hydrogenation, only platinum, palladium, rhodium, ruthenium and nickel are currently used. In addition some other elements and compounds were found useful for catalytic hydrogenation copper (to a very limited extent), oxides of copper and zinc combined with chromium oxide, rhenium heptoxide, heptasulfide and heptaselen-ide, and sulfides of cobalt, molybdenum and tungsten. 

Very recently, Hu et al. claimed to have discovered a convenient procedure for the aerobic oxidation of primary and secondary alcohols utilizing a TEMPO based catalyst system free of any transition metal co-catalyst (21). These authors employed a mixture of TEMPO (1 mol%), sodium nitrite (4-8 mol%) and bromine (4 mol%) as an active catalyst system. The oxidation took place at temperatures between 80-100 °C and at air pressure of 4 bars. However, this process was only successful with activated alcohols. With benzyl alcohol, quantitative conversion to benzaldehyde was achieved after a 1-2 hour reaction. With non-activated aliphatic alcohols (such as 1-octanol) or cyclic alcohols (cyclohexanol), the air pressure needed to be raised to 9 bar and a 4-5 hour of reaction was necessary to reach complete conversion. Unfortunately, this new oxidation procedure also depends on the use of dichloromethane as a solvent. In addition, the elemental bromine used as a cocatalyst is rather difficult to handle on a technical scale because of its high vapor pressure, toxicity and severe corrosion problems. Other disadvantages of this system are the rather low substrate concentration in the solvent and the observed formation of bromination by-products. 

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