Nickel atom exchange

The presence of a remote C=C double bond in the alkyl halide was critical saturated analogs underwent nickel-catalyzed halogen-zinc exchange.248 It has been suggested that the double bond coordinates to the nickel atom. As a 7r-acceptor, the C=C bond removes some electron density from the metal, thus facilitating the reductive coupling (Scheme 154).246... 

M. Che, M. Richard, and D. OUvier ferromagnetic resonance study of dispersed nickel particles prepared by reduction of nickel ion-exchanged X-zeolites by hydrogen molecules or hydrogen atom beams, J. Chem. Soc. Faraday Trans. 176,1526-1534 (1980). 

The structure and properties of the initial nickel atom-propene complex has received detailed attention by Skell. The use of isotopically labeled propenes demonstrated that exchange occurs between the 1- and 3-positions but not between either the 1- and 2- or 2- and 3-positions (10) ... 

The non-relativistic DV-Xa calculation [21] was performed with the Slater exchange parameter, a = 0.7, for all atoms and with 50,000 DV sampling points, which provided a precision of less than 0.1 eV for valence electron energy eigenvalues. We employed the basis functions of the central nickel atom ls-4p orbitals, while those of the nitrogen and carbon atoms were used ls-2p orbitals. The calculations were carried out self-consistently until the difference in the orbital populations between the initial and final states of the iteration was less... 

As described in Chapter 3, the reasons given to explain this dilution effect of copper on nickel were that the hydrogenolysis reaction required a group or ensemble of nickel atoms on the catalyst surface and the presence of copper prevented the formation of the appropriately sized ensembles and the reaction was inhibited. On the other hand, the reactions involving C-H bond breaking take place on single atom sites so the surface dilution by copper has no effect until the surface is almost completely covered by the copper. The rate increase observed in the deuterium exchange on cyclopentane (Fig. 12.7b), however, is not easily rationalized by this surface ensemble effect alone. 

The final location of the displaced nickel atoms when the exchange structure is formed at the surface is now evident. When one-half of the nickel atoms in the (100) surface monolayer are replaced by oxygen atoms, these replaced atoms should form on the surface into patches totaling one-half monolayer, since alternate (100) planes are composed entirely of nickel atoms. This does not mean that these surface nickel atoms are non reactive for further oxygen adsorption. The LEED observations of an expanded lattice of 2-5% for the exchange structure on the (100) face is in agreement with the observation of an expanded lattice of 2.3% by Alessandrini and Freedman. 

Observations on (110) nickel (3i, 33) show the presence of a 1 x 3 structure which occurs between the 1x2 structure and NiO. This 1x3 structure appears to consist of a surface exchange structure having two adjacent rows of oxygen atoms alternating with one row of nickel atoms. It is not known at present if there is a three dimensional structure associated with this. 

At the same time that our work on ethane hydrogenolysis and cyclohexane dehydrogenation on nickel-copper alloys was published, a paper by Ponec and Sachtler on the reactions of cyclopentane with deuterium appeared (9). These workers reported data on the rates of formation of deuterocyclopen-tanes via exchange, and of CD4 by hydrogenolysis. The exchange reaction occurred at about the same rate (per surface nickel atom) on nickel-copper alloys as on pure nickel, while the rate of formation of CD4 was substantially decreased. 

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