Transition metal substrates

The reaction of [Me2NCCl2]Cl with various transition metal substrates has proved to be a useful method of synthesizing chloroaminocarbene complexes (121). A Group 8 example is... 

The purpose of this article is to review the results of transient low pressure studies of carbon monoxide oxidation over transition metal substrates. Particular emphasis is given to the use of in-situ electron spectroscopy, flash desorption, modulated beam and titration techniques. The strengths and weaknesses of these will be assessed with regard to kinetic insight and quantification. An attempt will be made to identify questions that are ripe for investigation. Although not limited to it, the presentation emphasizes our own work. A very recent review of the carbon monoxide oxidation reaction C l) will be useful to readers who are interested in a more comprehensive view. 

Attempts to produce vinylidene in the free state result in rapid reversion to ethyne, with a lifetime of 10 ° s [1]. As with many reactive organic intermediates, however, vinylidene can be stabilized by complexation to a metal center, using the lone pair for coordination and thus preventing the reversion to ethyne. Most 1-aIkynes can be converted into the analogous vinylidene complexes by simple reactions with appropriate transition metal substrates (Equation 1.2) ... 

Palladium black is another transition metal subsTrate on which hydrogen chemisorption has been studied by inelastic neutron scattering (33). Two bands were observed at 916 cm-1 and 823 cm"1 which were assigned to vibrations of the hydrogen parallel and perpendicular to the surface, respectively. A bridge site was inferred based on the number of modes observed without making any assumptions about the expected excitation frequencies for the different site geometries. 

The first successful syntheses utilizing trifluoromethyl iodide in transition metal chemistry were reported by Stone and his students. Stone reasoned that if CF3I would not react with transition metal anions to form trifluoromethyl derivatives [see Eq. (3)] then perhaps compounds containing perfluoroalkyl substituents could be generated by the oxidative addition of perfluoroalkyl halides to low valent transition metal substrates (9,10). The first reported trifluoromethyl-substi-tuted transition metal complex prepared by this route is shown in Eq. (4) (41). 

As indicated in Table II, the complexes examined thus far have all contained coordinatively unsaturated dH or d10 metal ions, as has most commonly been the case in oxidative addition reactions of alkyl halides with transition metal substrates. Almost all of the products of these reactions are immediately recognizable as having arisen from an oxidative addition reaction, but in some instances the species isolated, e.g., CF3Au(P3) (59), (CF3)2Pt(COD) (54), and CF3Pt(PEt3)2I (55), were found to be in the same oxidation state as the reagents that had been originally employed. 

Other reactions of CF3I with transition metal substrates that have... 

Treatment of tantalum hydride complexes with hexamethylsilacyclopropane at 65 °C in the presence of a Lewis base gave the corresponding silyl-tantalum compounds in excellent yield. It appears that this reaction takes place via direct insertion of dimethylsilylene into the tantalum-hydride bond, which would make it the first example of silylene trapping by a transition-metal substrate (Equation (15)) <87JA6210>. 

During this reaction, the results of which are quite typical of those obtained from very early (electron poor) transition metal substrates, the only product isolated was the fluoride, which was separated in good yield. The only evidence for the existence of a Hf—CF3 intermediate was a small transient 19F NMR signal in the M—CF3 chemical shift region (33). 

In this context, rare earths on transition metal substrates attracted considerable research attention from two directions i) to understand the overlayer growth mechanisms involved [3] and ii) to prepare oxide-supported metal catalysts from bimetallic alloy precursor compounds grown in situ on the surface of a specific substrate [4,5]. The later studies are especially significant in terms of understanding the chemistry and catalytic properties of rare earth systems which are increasingly used in methanol synthesis, ammonia synthesis etc. In this paper, we shall examine the mechanism of Sm overlayer and alloy formation with Ru and their chemisorption properties using CO as a probe molecule. 

X. Chen, and J. Narayan, Effect of the chemical nature of transition-metal substrates on chemical-vapor deposition of diamond, y. Appl. Phys., 74 (6) 4168-4173 (1993)... 

By way of example, we show how the transition-state rate expression can be used to determine the rates for both surface desorption and the dissociation of CO at low surface coverage on the terrace sites of the transition-metal substrate. This also allows for an illustration of the concepts of tight and loose transition states and their respective definitions[ ff l. 

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