Activation of the Hydrogen Molecule

When activated by metallic catalysts, hydrogen may be transferred from the metallic center to unsaturatcd organic molecules. The nature and reactivity of transition metal hydrides depend on the central metals as well as on the electronic and steric properties of the ligands. Metal hydrides with optically active ligands are chiral and thus, are capable of asymmetric hydrogenation. 

SCHEME 1. Activation of hydrogen molecules by transition metals. 

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In many respects, aqueous organometallic hydrogenations do not differ from the analogous reactions in organic solvents. There are, however, three important points to consider. One of them concerns the activation of the hydrogen molecule [3]. The basic steps are the same in both kinds of solvents, i.e. Hi can be split either by homolysis or heterolysis, equations (3.1) and (3.2), respectively. 

In the following sections a number of general types of reactions recur constantly. These are summarized here. The first three most commonly involve activation of the hydrogen molecule. 

The comparison of TPR-MS diagrams reported in Figure 4.4 (a) and also allows us to conclude that, the dispersed rhodium phase, enhances the reducibility of terbia, thus suggesting that, under the investigated conditions the reduction process is controlled by the activation of the hydrogen molecule. 

J. Horiuti (Hokkaido University, Sapporo, Japan) I might suggest that the activation of the hydrogen molecule by a metal ion, e.g., cupric ion. 

The activation of the hydrogen molecule occurs as a result of its homolytic or heterolytic splitting ... 

By increasing the electrical energy in a fixed amount of gas, the temperature is raised and may reach 5000°C or higher.P i Such high temperatures produce an almost complete dissociation of the hydrogen molecules, the CH radicals, and other active carbon species. From this standpoint, arc-plasma deposition has an advantage over microwave-plasma or thermal CVD since these produce much less atomic hydrogen. 

Fignre 5.12 shows the structure of the activated hydrogen molecnle. The Pt catalyst may canse the increase of activation of the hydrogen molecnle. As mentioned before, since the activated hydrogen molecnles are more likely to be attracted each other than normal hydrogen molecules in the reaction medium, these attractions may result in liquefaction of hydrogen. 

In these solvents heterolytic activation can occur through a chain of hydrogen-bonded solvent molecules connecting the H2 proton with the basic center (for instance via a six-membered transition state) instead of an -bond metathesis [2+2] of the hydrogen molecule through a four-membered transition state (Scheme 2). 

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