Metal atom migration

Grubbs and Brunck found that ethene was formed and that this product consisted of 88% C2H3D, 6% C2H4, and 6% symmetric C2H2D2. They also demonstrated that C2H4 and C2H2D2 were not formed by a secondary reaction. From this they inferred that a migration of 12% of the metal atoms had taken place. 

Anodic dissolution reactions of metals typically have rates that depend strongly on solution composition, particularly on the anion type and concentration (Kolotyrkin, 1959). The rates increase upon addition of surface-active anions. It follows that the first step in anodic metal dissolution reactions is that of adsorption of an anion and chemical bond formation with a metal atom. This bonding facilitates subsequent steps in which the metal atom (ion) is tom from the lattice and solvated. The adsorption step may be associated with simultaneous surface migration of the dissolving atom to a more favorable position (e.g., from position 3 to position 1 in Fig. 14.1 la), where the formation of adsorption and solvation bonds is facilitated. 

We have also observed competition between products resulting from C-C and C-H bond activation in reactions of Y with propene,138 propyne,143 2-butyric,143 four butene isomers,138 acetaldehyde,128 acetone,128 ketene,144 and two cyclohexadiene isomers,145 as well as for Zr, Nb, Mo, and Mo with 2-butyne.143 In this chapter, we use the term C-C activation to describe any reaction leading to C-C bond fission in which the hydrocarbon reactant is broken into two smaller hydrocarbon products, with one hydrocarbon bound to the metal. It is important to note, however, that C-C activation does not necessarily require true C-C insertion. As will be shown in this chapter, the reaction of Y, the simplest second-row transition metal atom, with propene leads to formation of YCH2 +C2H4. The mechanism involves addition to the C=C bond followed by H atom migration and C-C bond fission, rather than by true C-C insertion. 

Like other metal reactions studied previously in our laboratory, H2 elimination is initiated by insertion into one of the C-H bonds forming HMC3H5. The reaction rate constant for Y + cyclopropane was found to be very small at room temperature, 0.7 x 10 12 cm3 s 1, and it was suggested that the reaction most likely involved termolecular stabilization of C-H or C-C insertion complexes, rather than molecular elimination.22 By analogy with other systems studied, the dynamically most favorable route to H2 loss in this case is likely via H atom migration to the Y-H moiety, with concerted... 

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

Most metals are covered by an oxide (impervious and insulating, or more often, as with Al, defect-ridden). In contrast, gold has no oxide, and has the advantage of making SAMs with thiols. However, Au atoms migrate somewhat after deposition to minimize total energy, and migrate even more under an electric field... 

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