Other Hydrides

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

The properties of water are seen to differ greatly from the other hydrides the deviations can be largely explained by the formation of hydrogen bonds between water molecules. 

The Pd-catalyzed hydrogenoiysis of acyl chlorides with hydrogen to give aldehydes is called the Rosenmund reduction. Rosenmund reduction catalyzed by supported Pd is explained by the formation of an acylpalladium complex and its hydrogenolysis[744]. Aldehydes can be obtained using other hydrides. For example, the Pd-catalyzed reaction of acyl halides with tin hydride gives aldehydes[745]. This is the tin Form of Rosenmund reduction. Aldehydes are i ormed by the reaction of the thio esters 873 with hydrosilanes[746,747]. 

Figure 3.6 Plots showing the high values of mp, bp and heat of vaporization of NH3, H2O and HF when compared with other hydrides. Note also that the mp of CH4 (—I82.5 C) is slightly higher than that of SiHi (-185X).

Other hydrides with interstitial or metallic properties are formed by V, Nb and Ta they are, however, very much less stable than the compounds we have been considering and have extensive ranges of composition. Chromium also forms a hydride, CrH, though this must be prepared electrolytically rather than by direct reaction of the metal with hydrogen. It has the anti-NiAs structure (p.. 555). Most other elements... 

So far, we have seen two sources of nucleophilic H (LAH and NaBHr). There are many, many other hydride reagents that have been prepared (some that are even more reactive than LAH, and others that are even milder than NaBHr), for example ... 

Pure material is said not to ignite in air unless the temperature be increased or the pressure reduced. Presence of other hydrides as impurities causes ignition always to occur on contact [1]. However, 99.95% pure material, even at concentrations down to 1% in hydrogen and/or nitrogen, ignites in contact with air unless emerging at... 

The catalytic cycle with Ni catalysts is generally similar. The essential difference is the deactivation process, which in this case occurs not via the formation of a precipitate of Ni°, but rather due to interception of the highly reactive Ni° species by any fortuitous oxidant, such as oxygen. As Ni11 is not so easily reduced to Ni° as Pdn is to Pd°, Ni-catalyzed systems often require the addition of a stoichiometric reducing agent (Zn, DIBAL-H, other hydride transfer agents, BuLi, etc.). 

TiCl4 promotes reduction of carbonyl groups or acetals with R3SiH or R3SnH (Scheme 24) Other hydride sources are also effective.91 ... 

However, PH3 is not the only hydride of phosphorus, and it is not the only product of this reaction. The other hydride of phosphorus is diphosphine, P2H4, which is produced in the reaction just shown. This compound is spontaneously flammable in air, and it ignites phosphine, which is also flammable. 

The graphite furnace system was originally developed by Andreae [712] when he found that the quartz cuvette gave only very poor sensitivity for germanium. This was attributed to the formation of GeO, a very stable diatomic species, at the relatively low temperatures of the quartz cuvette. At the higher temperatures available with the graphite furnace (2600 °C for the determination of Ge), a sensitivity could be obtained for germanium comparable to that of the other hydride elements. 

The anomalously high boiling points of the hydrides NH3, H20 and HF, compared with the other hydrides in groups 5, 6 and 7, are explained... 

Ionic hydrogenations of C=C and C=0 bonds were reported prior to the development of ionic hydrogenations mediated or catalyzed by transition metals. Tri-fluoroacetic acid (CF3C02H) as the proton donor and triethylsilane (HSiEt3) as the hydride donor are most commonly used, though a variety of other acids and several other hydride donors have also been shown to be effective. A review [1] by Kursanov et al. of the applications of ionic hydrogenations in organic synthe-... 

Because of its high thermal stability compared to that of other hydrides, water does not decompose extensively below 2000 °K. Thus, at one atmosphere and 2500 °K it is only dissociated to the extent of 9 %. Accordingly, it is impossible to study the homogeneous decomposition by classical methods. It is only with the shock tube technique that the rates of pyrolysis of water and heavy water have been measured. 

All four dissolution procedures studied were found to be suitable for arsenic determinations in biological marine samples, but only one (potassium hydroxide fusion) yielded accurate results for antimony in marine sediments and only two (sodium hydroxide fusion or a nitricperchloric-hydrofluoric acid digestion in sealed Teflon vessels) were appropriate for determination of selenium in marine sediments. Thus, the development of a single procedure for the simultaneous determination of arsenic, antimony and selenium (and perhaps other hydride-forming elements) in marine materials by hydride generation inductively coupled plasma atomic emission spectrometry requires careful consideration not only of the oxidation-reduction chemistry of these elements and its influence on the hydride generation process but also of the chemistry of dissolution of these elements. 

Other hydride systems do not have such weight penalties and include magnesium nickel alloys, non-metallic polymers, or liquid hydride systems that use engine heat to disassociate fuels like methanol into a mixture of hydrogen and carbon monoxide. 

HCl and HF [56] with increasing values in a range [0.10, 0.45] respectively a negative sign and large magnitudes are common to other hydrides of group 13, for which calculations yield values —8.207 for [96] and —3.370 for... 

With the beginning of the 1990s the interest in hydrides was somehow refocused. Firstly, interest evolved in other hydrides of Mg with transition metals, beyond already well-researched Mg Ni stoichiometry. Mg and Fe do not alloy to form... 

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