Nickel hydride, decomposition

As far as hydride decomposition is concerned, the relations are reversed. The larger the metal crystals are the slower their hydride decomposes (62). Moreover some deposits situated on the exit points of dislocations, for example on the surface of a nickel hydride crystal, inhibit hydrogen desorption and result in prolonging the hydride existence in the crystal (87). 

Baranowski [680] concluded that the decomposition of nickel hydride was rate-limited by a volume diffusion process the first-order equation [eqn. (15)] was obeyed and E = 56 kJ mole-1. Later, Pielaszek [681], using volumetric and X-ray diffraction measurements, concluded from observations of the effect of copper deposited at dislocations that transportation was not restricted to imperfect zones of the crystal but also occurred by diffusion from non-defective regions. The role of nickel hydride in catalytic processes has been reviewed [663]. 

Mechanistic studies, based on stoichiometric reactions, revealed that the transient nickel hydride (Cp -NHC)NiH complex, generated by reaction of (Cp -NHC)Ni(OBu ) with phenylsilane, is the active species in the hydrosilylation of aldehydes. The identity of (Cp -NHC)NiH was demonstrated by NMR spectroscopy (the characteristic hydride resonance appeared at -18 ppm). Surprisingly, the Ni-H is fairly stable in solution no decomposition is observed after being kept in solution for... 

Ni3C decomposition is included in this class on the basis of Doremieux s conclusion [669] that the slow step is the combination of carbon atoms on reactant surfaces. The reaction (543—613 K) obeyed first-order [eqn. (15)] kinetics. The rate was not significantly different in nitrogen and, unlike the hydrides and nitrides, the mobile lattice constituent was not volatilized but deposited as amorphous carbon. The mechanism suggested is that carbon diffuses from within the structure to a surface where combination occurs. When carbon concentration within the crystal has been decreased sufficiently, nuclei of nickel metal are formed and thereafter reaction proceeds through boundary displacement. 

In a study of thermal stability and hydrogen sorption characteristics of a series of sorbent tablets composed of hydride-forming metals dispersed in polymers under a 50% hydrogen in argon atmosphere, it was found that tablets of 80% palladium in PTFE, and 80% of 1 5 atom lanthanum-nickel alloy in PTFE could not be used above 247° C because of explosive decomposition of the PTFE. 

Exercise 31-12 7r-Propenyl(ethyl)nickel decomposes at —70° to give propene and ethene. If the ethyl group is labeled with deuterium as —CH2—CD3, the products are C3H5D and CD2=CH2. If it is labeled as —CD2—CH3, the products are C3H6 + CD2=CH2. Are these the products expected of a radical decomposition, or of a reversible hydride-shift followed by decomposition as in the mechanism of Section 31-2B Suppose the hydride-shift step were not reversible, what products would you expect then ... 

Various active nickel catalysts obtained not via reduction of nickel oxide with hydrogen have been described in the literature. Among these are the catalysts obtained by the decomposition of nickel carbonyl 10 by thermal decomposition of nickel formate or oxalate 11 by treating Ni-Si alloy or, more commonly, Ni-Al alloy with caustic alkali (or with heated water or steam) (Raney Ni) 12 by reducing nickel salts with a more electropositive metal,13 particularly by zinc dust followed by activation with an alkali or acid (Urushibara Ni) 14-16 and by reducing nickel salts with sodium boro-hydride (Ni boride catalyst)17-19 or other reducing agents.20-24... 

Tessier, P. and Akiba, E. (2000) Decomposition of nickel-doped magnesium hydride prepared by reactive mechanical alloying. Journal of Alloys and Compounds, 302, 215-217. 

The present preparation Illustrates a general and convenient method for a two-step deoxygenation of carbonyl compounds to olefins. Related procedures comprise the basic decomposition of p-toluenesulfonylhydrazones,2 the hydride reduction of enol ethers,3 enol acetates,9 enamines,3 3 the reduction of enol phosphates (and/or enol phosphorodlamidates) by lithium metal in ethylamine (or liquid ammonia),33 the reduction of enol phosphates by titanium metal under aprotic conditions,32 the reduction of thioketals by Raney nickel,33 and the reduction of vinyl sulfides by Raney nickel in the presence of isopropylmagnesium bromide.3 ... 

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