Metal-hydrides

Metal hydride trapping agents have been used extensively in studying the reaction of alkyl radicals with monomers.489 400 

A consequence of the selectivity for electrophilic radicals is that not all products are trapped with equal efficiency. With electron-rich monomers (e.g. S) oligomerization may complicate analysis. Other possible complications in the utilization of this method have been discussed by Russell.491 

Metal hydrides are important because the M-H bond enters into so many key reactions, such as undergoing insertion with a wide variety of C=X bonds to give either stable species or reaction intermediates with M-C bonds, often as part of a catalytic cycle. 

Hieber s 1931 claim that H2Fe(CO)4 contains two Fe-H bonds long remained controversial as late as 1950, Sidgwick still preferred the incorrect (CO)2Fe(COH)2 structure. Only with the discovery of Cp2ReH, PtHCl(PR3)2, and the striking polyhydride K2[ReHg] in the period 1955-1964, did the reality of the M-H bond as a normal covalency become widely accepted. The landmark discovery of molecular hydrogen complexes, L M-(H2), emphasizes the remarkably rich chemistry of the simplest atom, H. 

Hydrides can be detected by H NMR spectroscopy because they resonate to high field of SiMe4 in an otherwise unoccupied spectral range (0 to —606). They couple with a suitable metal and with cis (J = 15-30 Hz) and trans (/ = 90-150 Hz) phosphines this cis/trans difference is often useful for determining the stereochemistry of the complex. Inequivalent hydrides also couple with each other (/ = 1-10 Hz). IR M-H stretching frequencies range from 1500 to 2200 cm but the intensities are often weak. 

X-rays are scattered by electron density, not by the atomic nuclei, so crystallographic detection of hydride ligands is hard since H has 

Protonation requires a basic metal complex, but the action of a main-group hydride on a metal halide is very general. The third route, oxidative addition, is of particular importance in catalysis. Finally, hydrides are formed by the (3 elimination of a variety of groups. 

Even though the first metal hydride was discovered more than 100 years ago, intensive research on this class of materials is relatively new. Investigation of the hydrogen-metal interactions is very interesting from a fundamental point of view as well as for practical applications. In this chapter, we focus on the materials and properties that are especially important for hydrogen storage applications. 

The hydrogen molecule first dissociates into its two atoms, which are chemisorbed on the surface of the metal/alloy and then diffuse into the bulk lattice. The dissolved atoms can take the form of a random solid solution or react to produce a hydride of fixed stoichiometric composition. The quantity of 

The relationship between the equilibrium dissociation pressure (P) of a hydride and the absolute temperature (T) is expressed by the van t Hoff equation, i.e.. 

It has been claimed that the requirement to supply heat to desorb the hydrogen from the alloy is a positive safety feature, compared with storage as pressurized gas or as liquid hydrogen, since desorption cannot happen spontaneously. Although true, this is a dubious argument because in the event of an accident, any ingress of air may lead to combustion of the finely-divided hydride. 

Several criteria have been established for the realization of a practical hydrogen storage bed  

This is a demanding set of conditions and, although many hundreds of alloys have been studied, none have yet been found to be fully satisfactory. 

Diisobutylaluminum hydride (i-Bu2AlH-H20) reduces alkynes to alkenes. L1A1H4 can also reduce triple bonds in unsaturated alcohols. 

Alkynes are reduced preferentially to alkenes with preferred Z-selectivity with molar mixtures of LiAlH4 and several transition metal chlorides such as TiCh. 

Completely stereospecific frans-reduction of acetylenic alcohols to -allyl alcohol is reported with sodium bis(2-methoxyethoxy)aluminium dihydride (SMEAH or Red-Al), where LiAlH4 in various solvents is less selective. 

Chan stereospecific reduction of acetylenic alcohol 6.27 gives E-allylic alcohol 6.28 by means of SMEAH in 83% yield. 

Hydrogen forms binary compounds of the type MH, or M Hy of varying stability with all the main group elements except helium and neon. All of the lanthanoids and actinoids that have been studied also 

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Transition metal hydrides. These are formed by hydrogen uptake by the metal. The phases are often non-stoicheiometric. 

Vidal V, Theolier A, Thivolle-Cazat and Basset J M 1997 Metathesis of alkanes catalyzed by silica-supported transition metal hydrides Soienoe 276 99-102... 

The hydrides of beryllium and magnesium are both largely covalent, magnesium hydride having a rutile (p. 36) structure, while beryllium hydride forms an electron-deficient chain structure. The bonding in these metal hydrides is not simple and requires an explanation which goes beyond the scope of this book. 

Boron forms a whole series of hydrides. The simplest of these is diborane, BjH. It may be prepared by the reduction of boron trichloride in ether by lithium aluminium hydride. This is a general method for the preparation of non-metallic hydrides. 

Metal hydrides reduce preferably polar double bonds, whereas catalytic hydrogenation is somewhat selective for non-polar double bonds. Selective protection of amino groups in amino acids. 

Gaylord, N. G, 1956, Reduction with Complex Metal Hydrides, Interscience New York... 

For most laboratory scale reductions of aldehydes and ketones catalytic hydro genation has been replaced by methods based on metal hydride reducing agents The two most common reagents are sodium borohydride and lithium aluminum hydride... 

Oxygen Acetaldehyde, acetone, alcohols, alkali metals, alkaline earth metals, Al-Ti alloys, ether, carbon disulflde, halocarbons, hydrocarbons, metal hydrides, 1,3,5-trioxane... 

Metal halide Metal hydride Metal hydrides... 

Mischmetal [62379-61-7] Misch metal [8049-20-5] Misch metal hydride Miscible blend Miscible liquids... 

Common catalyst compositions contain oxides or ionic forms of platinum, nickel, copper, cobalt, or palladium which are often present as mixtures of more than one metal. Metal hydrides, such as lithium aluminum hydride [16853-85-3] or sodium borohydride [16940-66-2] can also be used to reduce aldehydes. Depending on additional functionahties that may be present in the aldehyde molecule, specialized reducing reagents such as trimethoxyalurninum hydride or alkylboranes (less reactive and more selective) may be used. Other less industrially significant reduction procedures such as the Clemmensen reduction or the modified Wolff-Kishner reduction exist as well. 

The reaction of metal hydrides and BF depends on the stoichiometry as well as the nature of the metal hydride. For example, LiH and BF may form diborane (6) orUthiumborohydride (31,32) ... 

Boron trifluoride is used for the preparation of boranes (see Boron compounds). Diborane is obtained from reaction with alkafl metal hydrides organoboranes are obtained with a suitable Grignard reagent. 

Although a few simple hydrides were known before the twentieth century, the field of hydride chemistry did not become active until around the time of World War II. Commerce in hydrides began in 1937 when Metal Hydrides Inc. used calcium hydride [7789-78-8J, CaH2, to produce transition-metal powders. After World War II, lithium aluminum hydride [16853-85-3] LiAlH, and sodium borohydride [16940-66-2] NaBH, gained rapid acceptance in organic synthesis. Commercial appHcations of hydrides have continued to grow, such that hydrides have become important industrial chemicals manufactured and used on a large scale. 

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

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

Potassium, Pubidium, and Cesium idjdrides. Although all the other alkah metal hydrides have been synthesized and some of the properties measured, only potassium hydride [7693-26-7] is commercially available. KH is manufactured in small amounts and sold as a mineral oil dispersion. It is a stronger base than NaH and is used to make the strong reducing agent KBH(C2H )2 and the super bases RNHK and ROK (6). 

Alkaline-Earth Metal Hydrides. Table 2 gives thermochemical data of alkaline-earth metal hydrides. AH form orthorhombic crystals. 

Trunsition-MetnlHydrides, Tiansition-metal hydiides, ie, inteistitial metal hydrides, have metalhc properties, conduct electricity, and ate less dense than the parent metal. Metal valence electrons are involved in both the hydrogen and metal bonds. Compositions can vary within limits and stoichiometry may not always be a simple numerical proportion. These hydrides are much harder and more brittie than the parent metal, and most have catalytic activity. 

Hydrogen-storage alloys (18,19) are commercially available from several companies in the United States, Japan, and Europe. A commercial use has been developed in rechargeable nickel—metal hydride batteries which are superior to nickel—cadmium batteries by virtue of improved capacity and elimination of the toxic metal cadmium (see BATTERIES, SECONDARYCELLS-ALKALINe). Other uses are expected to develop in nonpolluting internal combustion engines and fuel cells (qv), heat pumps and refrigerators, and electric utility peak-load shaving. 

R. Bau, "Transition Metal Hydrides," ddvances in Chemistry Series, Vol. 167, American Chemical Society, Washington, D.C., 1978. 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

One of the problems with early hydride systems was decrepitation of the alloy. Each time the metal hydride storage tank was recharged the particles would break down and eventually the particles became so small that they began to pass through the 5-p.m sintered metal filter which kept the hydride inside the tank. Addition of 0.5% manganese, which caused the decrepitation process to cease once the particles reached a size of about 10 p.m, solved this problem. 

One of the principal advantages of hydrides for hydrogen storage is safety (25). As part of a study to determine the safety of the iron—titanium—manganese metal hydride storage system, tests were conducted in conjunction with the U.S. Army (26). These tests simulated the worst possible conditions resulting from a serious coUision and demonstrated that the metal hydride vessels do not explode. 

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