Hydrogen hydridic” model

The diversity of the substrates, catalysts, and reducing methods made it difficult to organize the material of this chapter. Thus, we have chosen an arrangement related to that used by Kaesz and Saillant [3] in their review on transition-metal hydrides - that is, we have classified the subject according to the applied reducing agents. Additional sections were devoted to the newer biomimetic and electrochemical reductions. Special attention was paid mainly to those methods which are of preparative value. Stoichiometric hydrogenations and model reactions will be discussed only in connection with the mechanisms. 

Exposure of silicon to atomic hydrogen increases the surface recombination velocity.111213 The free energy of formation of SiH4, the most stable of the hydrides of silicon, is only — lOKcal/mole. Since four electron pairs are shared in the formation of the molecule, the free energy of formations per Si-H bond is only -2.5 Kcal or about O.leV. Because of the weak chemisorption, heating of the silicon to temperatures above 500 C is adequate to release the hydrogen. Our model explains the increase in surface recombination velocity by the weak chemisorption of hydrogen, which may increase the density of surface states within the band gap (see Fig. 2b). 

Keywords hydride, hydrogen, hydride bed, heat pump, modeling, experiment. 

Metallic hydrides are usually nonstoichiometric compounds, as expected from their relatively low heats of formation and the mobility of hydrogen. They are ordinarily described, chemically, in terms of any of three models in which hydrogen is considered a small interstitial atom, a proton, or a hydride anion. These models are discussed critically with particular reference to the group V metal hydrides. The interstitial atom model is shown to be useful crystallographically, the protonic model is questioned, and the hydridic model is shown to be the most useful at present. The effect of hydrogen content on the lattice parameter of VHn and the structural and magnetic properties of several hydrides are discussed in terms of these models. 

It might be said in extenuation of the hydridic model that, according to the ideas of Kimball (19), the H atom should enter a more or less spherical electron cloud representing an unpaired electron associated with a metal atom. Thus, LiH is represented in Kimball s theory as a pair of tangent electron cloud spheres, or spheroids, each comprising two electrons of opposite spin centered about a +3 and a +1 nucleus, respectively. This picture is equally applicable to the hydrogen in CH4, HC1, or a metallic hydride—i.e., in all cases hydrogen is surrounded by a pair of electrons. 

Bulky H should not diffuse or show marked oscillational movement as indicated by magnetic resonance studies. The hydridic model actually provides a reasonable explanation for the mean amplitude of H vibrations (ca. 0.2 A.) and is noncommittal about diffusion. Conceivably, the barrier to diffusion comprising an Is2- configuration about the proton is in effect lowered by the distance of the barrier from the mean position of the nucleus. If the movement of hydrogen is quasitautomeric—for example, in keto-enol tautomerism—one may consider that it moves from one potential well to another as a proton. 

The relation of H content, n, to lattice parameter for VH is interpreted readily by the hydridic model as indicated in Figure 5, which represents a portion of a hypothetical unit cell in which H-, of radius 1.22 A., is located in an octahedral site in a BCC V+5 cell. The radius of V+5 is 0.48 A., both ionic radii being corrected for fourfold coordination (12). The V-H distance is, of course, the same as that given by the atomic model shown on the right, where the metal and hydrogen radii are, respectively, 0.93 and 0.56 A. (see also Figure 6). 

In terms of kinetics and mechanisms, electroless deposition processes have many similarities. In an attempt to analyze the electroless deposition, several mechanisms such as atomic hydrogen, hydride ion, metal hydroxide, electrochemical, and universal have been proposed.1-3 It is important to note that these mechanisms were developed for cases of nickel and copper electroless deposition, which were the most widely studied metals in this respect. Based on the proposed mechanisms, most of the features of electroless deposition can be explained. However, there are some characteristics of electroless deposition, which cannot be explained using these mechanisms. The major problems arise when attempting to generalize the proposed models explaining the mechanistic aspects. 

Particularly in the 1970s, several lines of evidence were taken to suggest a major role for electron-transfer processes in model reactions for the action of nicotinamide cofactors. Bruice and his coworkers [30, 36-38] in 1982-1984 showed that subtle effects rendered these observations deceptive, and that in fact hydride transfer is the only mechanism at work in the aqueous-solution hydrogen-transfer models that had formed the earlier focus. Further relevant references and an extraordinary analysis are given in the review by Westheimer [4]. The main outlines are discussed below. 

For many of these hydrides there is still discussion whether properties such as magnetic susceptibility, electrical conductivity, etc., are best accounted for by a hydridic model with M"+ and H- ions, a protonic model where the hydrogen electrons are lost to conduction bands in the metal, or an alloy-like model without appreciable charge separation. 

We will start with a very simple homopolymerization model that includes only initiation, propagation, transfer to hydrogen, -hydride elimination and imimolecular catalyst deactivation, as depicted in Table 2.4. From our previous discussion of the standard model for polymerization with coordination catalysts, it is known that several steps are not included in Table 2.4. It will be shown, however, that general expressions for population balances and the methods of moments starting with this simplified mechanism can be developed and later they can be extended, rather easily, to include more polymerization steps. 

This kind of compound is worthy of special attention on the part of theoretical chemists for several reasons. On the one hand, their identification is complicated, because of the problems associated with the experimental localization of hydrogen atoms bonded to metallic atoms. On the other hand, experimental observation of the interchange reaction between the molecular H2 and the hydrogen hydride shows the power of these compounds as models for the heterolytic activation reaction. 

In a sense the formation of t) -H2 complexes can be thought of as an intermediate stage in the oxidative addition of H2 to form two M-H bonds and, as such, the complexes might serve as a model for this process and for catalytic hydrogenation reactions by metal hydrides. Indeed, intermediate cases between and... 

The proposed mechanism of H2 evolution by a model of [FeFeJ-hydrogenases based upon DFT calculations [204-206] and a hybrid quanmm mechanical and molecular mechanical (QM/MM) investigation is summarized in Scheme 63 [207]. Complex I is converted into II by both protonation and reduction. Migration of the proton on the N atom to the Fe center in II produces the hydride complex III, and then protonation affords IV. In the next step, two pathways are conceivable. One is that the molecular hydrogen complex VI is synthesized by proton transfer and subsequent reduction (Path a). The other proposed by De Gioia, Ryde, and coworkers [207] is that the reduction of IV affords VI via the terminal hydride complex V (Path b). Dehydrogenation from VI regenerates I. 

Reactions leading to the formation of the catalytically active nickel hydride species from organonickel precursors (Section III) can be regarded as model reactions for olefin oligomerization reactions. The reactions described by Eq. (8) and Scheme 3 (Section III) show that RNiX compounds (R = methyl orallyl, X = halide or acetylacetonate) activated by Lewis acids add to double bonds under mild reaction conditions (-40° or 0°C). It follows further from these reactions that under conditions leading to olefin dimerization a rapid nickel hydride /3-hydrogen elimination reaction occurs. The fact that products resulting from olefin insertion into the nickel-carbon bond are only observed when /3-hydride... 

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