Alkali-Metal Hydrides

Alkali Metal Hydrides. Physical properties of the alkaU metal hydrides are given in Table 1. 

Table 1. Physical Properties of Alkali Metal Hydrides...

No binary hydrides have been characterized, but reactions of the metal powders with alkali metal hydrides in a hydrogen atmosphere lead to Li3RhH4 (planar RhH4 ) and M3MH6 (M = Li, Na M = Rh, Ir) with octahedral MHj [34],... 

The saline hydrides are white, high-melting-point solids with crystal structures that resemble those of the corresponding halides. The alkali metal hydrides, for instance, have the rock-salt structure (Fig. 5.39). 

War research forced us to explore new synthetic routes and we discovered the alkali metal hydride route to diborane. This solved the synthetic problem. At the same time we discovered sodium borohydride and developed simple synthetic methods for its preparation and manufacture. 

Brown, H. C., The Reactions of Alkali Metal Hydrides and Boro-hydrides with Lewis Acids of Boron and Aluminum, Congr. Lect., 17th Int. Congr. Pure Appl. Chem. p. 167. Butterworths, London, 1960. 

The interstitial hydrides of transition metals differ from the salt-like hydrides of the alkali and alkaline-earth metals MH and MH2, as can be seen from their densities. While the latter have higher densities than the metals, the transition metal hydrides have expanded metal lattices. Furthermore, the transition metal hydrides exhibit metallic luster and are semiconducting. Alkali metal hydrides have NaCl structure MgH2 has rutile structure. 

Perhaps the most depressing fact associated with the consequences of the above division is the lack of consistency often found in treatments of compounds which are essentially isostructural. Take, for instance, the different descriptions of the bonding situation in B2H6 on the one hand, and the isostructural (e.g. AI2CI6) molecules on the other while the latter may be treated by the conventional bonding principles expressed in Hyps. III.l to III.5, the treatment of the former (in terms of 3-centre bonds) breaks with Hyps. III.l to III.4. A similar conclusion is in fact reached in the majority of abnormal cases. Other simple examples are provided by the alkali-metal hydrides (with NaCl-type structure), CuH (with ZnS-wurtzite type structure), etc. These examples are typical in that it is only when a scarcity of electrons and/or orbitals enforces a search for extraordinary bonding principles that Hyps. III.l to III.4 are reluctantly (partly or completely) replaced by alter-... 

The synthesis of alkali metal organophosphides and arsenides is usually most conveniently achieved by the direct metalation of a primary or secondary phosphine/arsine with a strong deprotonating agent such as an alkyllithium or an alkali metal hydride ... 

Alkali-immobile dye-releasing quinone compounds, 19 293-294 Alkali lignins, 15 19-20 Alkali manganate(VI) salts, 15 596 Alkali manganates(V), 15 592 Alkali-metal alkoxide catalysts, 10 491 Alkali-metal alkoxides, effects of, 14 252 Alkali-metal alkylstannonates, 24 824 Alkali-metal fluoroxenates, 17 329-330 Alkali-metal hydrides, 13 608 Alkali-metal hydroxides, carbonyl sulfide reaction with, 23 622 Alkali-metal metatungstates, 25 383 Alkali-metal perchlorates, 18 211 Alkali-metal peroxides, 16 393... 

Strong bases such as alkali metal hydrides deprotonate B5H9 at —78 °C in ether solution to the octahydropentaborate(l—) which not only is a strong base but also an important reagent for the synthesis of derivatives [52]. The anion itself is not... 

An entry into the chemistry of mdo-decaborates is provided by utdo-B oH 4. As already mentioned, decaborane BioHi4 is easily deprotonated even in aqueous medium to yellow B10Hb and with strong bases, such as alkali metal hydrides, even further to the colorless anion B10H122-. This is summarized in the reversible reactions shown in Eq. (59). 

Phase-transfer catalytic conditions provide an extremely powerful alternative to the use of alkali metal hydrides for the synthesis of cyclopropanes via the reaction of dimethyloxosulphonium methylides with electron-deficient alkenes [e.g. 54-56] reaction rates are increased ca. 20-fold, while retaining high yields (86-95%). Dimethylphenacylsulphonium bromide reacts in an analogous manner with vinyl-sulphones [57] and with chalcones [58] and trimethylsulphonium iodide reacts with Schiff bases and hydrazones producing aziridines [59]. 

On looking into the literature to see whether this value of d(ff) = 0.28 A is meaningful, it was found [1] that this is the value suggested by Pauhng [8] to account for the radius of H in the partially ioitic bonds in hydrogen halides and in alkali metal hydrides The author then used this value of d(H+) = 0.28 A to estimate the radii of alkali metal ions from the observed bond distances d(MH) j in the metal hydrides, MH. It was a pleasant surprise to find that,... 

Zaluski, L., Zaluska, A., Strom-Olsen, J.O. 1999. Hydrogenation properties of complex alkali metal hydrides fabricated by mechano-chemical synthesis. J Alloys Comp 290 71-78. 

Such reactions have been observed to occnr where an alanate snch as NaAlH is milled with alkali metal hydride, such as NaH or LiH. 

Elansari et al. [201] developed a novel method of synthesizing alkali metal hydrides Na, KH, RbH, and CsH by reactive mechanical milling of pure alkaline metals under hydrogen pressure up to 30 bars in a planetary mill (Retsch PM 400). The reaction proceeds in 16 h and gives 3-15 g of very pure alkali metal hydride with FCC crystal structure (space group Fm3m). 

G. Sandrock, J. Reilly, J. Graetz, W.M. Zhou, J. Johnson, J. Wegrzyn, Alkali metal hydride doping of a-AlHj for enhanced desorption kinetics, J. Alloys Compd. 421 (2006) 185-189. 

L. Elansari, L. Antoine, R. Janot, J.C. Gachon, J.J. Kuntz, D. Guerard, Preparation of alkali metal hydrides by mechanical alloying, J. Alloys Compd. 329 (2001) L5-L8. 

Sulfonation of polybenzimidazole was also accomplished by proton abstraction with an alkali metal hydride followed by reaction with sodium (4-bromomethyl)benzenesulfonate. The degree of sulfonation in this synthesis can be controlled by the... 

Catalysts and reaction conditions used are generally similar to those used for olefin isomerization. Catalysts reported are sodium-organosodium catalysts prepared in situ by reaction of a promoter such as o-chloro-toluene or anthracene with sodium 19-24), alkali metal hydrides 20,21), alkali metals 22), benzylsodium 26), and potassium-graphite 26). These catalysts are strong bases that can react with alkylaromatics to replace a benzylic hydrogen [Reaction (2)]. 

Alkyl chlorides are with a few exceptions not reduced by mild catalytic hydrogenation over platinum [502], rhodium [40] and nickel [63], even in the presence of alkali. Metal hydrides and complex hydrides are used more successfully various lithium aluminum hydrides [506, 507], lithium copper hydrides [501], sodium borohydride [504, 505], and especially different tin hydrides (stannanes) [503,508,509,510] are the reagents of choice for selective replacement of halogen in the presence of other functional groups. In some cases the reduction is stereoselective. Both cis- and rrunj-9-chlorodecaIin, on reductions with triphenylstannane or dibutylstannane, gave predominantly trani-decalin [509]. 

These same arguments are now applied to the alkali-metal hydrides. The following Table shows the nearest-neighbour distances 24) and the anion-radius calculated on the assumption of anion-cation and anion-anion contact if one consistently assumes anion-anion contact in the... 

Alkali metal hydrides react with diborane to form metal borohydrides B2He+2NaH 2NaBH4... 

Alkali metal hydrides reduce boron trichloride to diborane at ordinary temperatures ... 

Lithium hydride and sodium hydride are the only alkali metal hydrides of much practical importance. They are useful when it is desirable for proton (or hydrogen atom) transfers to accompany electron-transfer events. Because these hydrides react quickly with water to form alkali metal hydroxides and hydrogen gas, they are frequently used as drying agents, particularly for hydrocarbons and ethers. Care should be exercised in using them to dry solvents that are not predried, and they should not be used to dry alcohols or halogenated solvents. 

Tab. 4 Standard free energies of formation and standard potentials for alkali metal hydrides...

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