Tetrahydroborates

The tetrahydroborates are a remarkable family of compounds, most of which have unusual chemical and stmctural characteristics. The aLkah metal tetrahydroborates (also known as borohydrides) are very ordinary, well-behaved, ionic compounds. The BH4 ion is isoelectronic with CH4 and NH4, and is about as interesting. It is a convenient source of the hydride ion, and as the alkali metal salts are soluble in water, they are valuable reducing agents. But so far as structural matters arc concerned, they are not very exciting. 

Part of the structure of [ Co(BH4) Ph2P(CH2)5PPh2 2] 0.5C6H6. The phenyl groups and all hydrogen atoms bonded to carbon are omitted. Redrawn with permission from [26]. Copyright 1985 American Chemical Society. 

NMR spectra of A1(BH4)3 (a) H spectrum, (b) H spectrum with decoupling, (c) B spectrum and (d) H spectrum with B decoupling. Redrawn from [30] with permission of The Royal Society of Chemistry. 

Mechanisms for exchange of terminal and bridge hydrogen atoms in A1(BH4)3. For clarity, only one BH4 group is shown in detail. 

Radial distribution curves for gaseous Be(BH4)2, obtained in separate experiments. Curve (a) is redrawn with permission from [32]. Copyright 1994 American Chemical Society. Curve (h) reprinted from [33] Copyright 1%8 Acta Chem Scand./RSC. Note particularly the absence of a peak near 3.6 A in curve (a). 

An endothermic (4.18kJ mol ) structural transition is observed at 118°C from the orthorhombic low-temperature structure to the hexagonal high-temperature structure. The high-temperature structure melts at 287 °C with a latent heat of 7.56 kJ mol . Both transitions are reversible, although a small hysteresis in temperature was observed. 

Harris and Meibohm suggested the space group Pcmn for UBH4 at room temperature, which turned out to be wrong [82]. The low- and high-temperature structures were investigated by means of synchrotron X-ray powder diffraction [83, 84]. 

Recently it has been shown that the hydrogen desorption reaction is reversible and the end products lithium hydride and boron absorb hydrogen at 690 ° C and 200 bar to 

The enthalpy diagram of LiBH4 shotvs the hydrogen desorption steps and also a possible path for the reabsorption of hydrogen. It is essential that three hydrogen atoms bind to boron and subsequently an is transferred from the LiH to the BH3  

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The elements listed in the table of Figure 15.2 are of importance as environmental contaminants, and their analysis in soils, water, seawater, foodstuffs and for forensic purposes is performed routinely. For these reasons, methods have been sought to analyze samples of these elements quickly and easily without significant prepreparation. One way to unlock these elements from their compounds or salts, in which form they are usually found, is to reduce them to their volatile hydrides through the use of acid and sodium tetrahydroborate (sodium borohydride), as shown in Equation 15.1 for sodium arsenite. 

A schematic illustration of a typical inlet apparatus for separating volatile hydrides from the analyte solution, in which they are generated upon reduction with sodium tetrahydroborate. When the mixed analyte solution containing volatile hydrides enters the main part of the gas/liquid separator, the volatiles are released and mix with argon sweep and makeup gas, with which they are transported to the center of the plasma. The unwanted analyte solution drains from the end of the gas/liquid separator. The actual construction details of these gas/liquid separators can vary considerably, but all serve the same purpose. In some of them, there can be an intermediate stage for removal of air and hydrogen from the hydrides before the latter are sent to the plasma. 

Some elements (S, Se, Te, P, As, Sb, Bi, Ge, Sn, Pb) are conveniently converted into their volatile hydrides before passed into the plasma. The formation of the hydrides by use of sodium tetrahydroborate (sodium borohydride) can be batchwise or continuous. 

Although the lUPAC has recommended the names tetrahydroborate, tetrahydroaluminate, etc, this nomenclature is not yet ia general use. Borohydrides. The alkaU metal borohydrides are the most important complex hydrides. They are ionic, white, crystalline, high melting soHds that are sensitive to moisture but not to oxygen. Group 13 (IIIA) and transition-metal borohydrides, on the other hand, are covalendy bonded and are either Hquids or sublimable soHds. The alkaline-earth borohydrides are iatermediate between these two extremes, and display some covalent character. 

Boron Triiodide. Boron ttiiodide is not manufactured on a large scale. Small-scale production of BI from boron and iodine is possible in the temperature range 700—900°C (70—72). Excess I2 can be removed as Snl by reaction with Sn, followed by distillation (71). The reaction of metal tetrahydroborates and I2 is convenient for laboratory preparation of BI (73,74). BI can also by synthesized from B2H and HI in a furnace at 250°C (75), or by the reaction of B with excess Agl or Cul between 450—700°C, under vacuum (76). High purity BI has been prepared by the reaction of I2 with mixtures of boron carbide and calcium carbide at elevated temperatures. 

Fig. 5. Modes of M—H—B bonding where M—H—B represents a three-center hydrogen bridge bond for (a), (b), (c) tetrahydroborates and for (d), (e), (f)...

Unsymmetrical cleavage of B2H by metal hydrides gives metal tetrahydroborate salts, also called metal borohydrides or hydroborates. 

Tetrahydroborates. The tetrahydroboranes constitute the most commercially important group of boron hydride compounds. Tetrahydroborates of most of the metals have been characterized and their preparations have been reviewed (46). The important commercial tetrahydroborates are those of the alkah metals. Some properties are given ia Table 4. 

Sodium tetrahydroborate is quite soluble ia Hquid ammonia and soluble to some extent ia a variety of other solvents. It is appreciably soluble only ia... 

Despite the fact that many boron hydride compounds possess unique chemical and physical properties, very few of these compounds have yet undergone significant commercial exploitation. This is largely owing to the extremely high cost of most boron hydride materials, which has discouraged development of all but the most exotic appHcations. Nevertheless, considerable commercial potential is foreseen for boron hydride materials if and when economical and rehable sources become available. Only the simplest of boron hydride compounds, most notably sodium tetrahydroborate, NajBHJ, diborane(6), B2H, and some of the borane adducts, eg, amine boranes, are now produced in significant commercial quantities. 

Sodium Tetrahydroborate, Na[BH ]. This air-stable white powder, commonly referred to as sodium borohydride, is the most widely commercialized boron hydride material. It is used in a variety of industrial processes including bleaching of paper pulp and clays, preparation and purification of organic chemicals and pharmaceuticals, textile dye reduction, recovery of valuable metals, wastewater treatment, and production of dithionite compounds. Sodium borohydride is produced in the United States by Morton International, Inc., the Alfa Division of Johnson Matthey, Inc., and Covan Limited, with Morton International supplying about 75% of market. More than six million pounds of this material suppHed as powder, pellets, and aqueous solution, were produced in 1990. 

Other tetrahydroborates of less commercial importance are lithium borobydride [16949-15-8], LiBH, and potassium borobydride [13762-51 -1],... 

Treatment of thiiranes with lithium aluminum hydride gives a thiolate ion formed by attack of hydride ion on the least hindered carbon atoms (76RCR25), The mechanism is 5n2, inversion occurring at the site of attack. Polymerization initiated by the thiolate ion is a side reaction and may even be the predominant reaction, e.g. with 2-phenoxymethylthiirane. Use of THF instead of ether as solvent is said to favor polymerization. Tetrahydroborates do not reduce the thiirane ring under mild conditions and can be used to reduce other functional groups in the presence of the episulfide. Sodium in ammonia reduces norbornene episulfide to the exo thiol. 

A flow-injection system with electrochemical hydride generation and atomic absorption detection for the determination of arsenic is described. This technique has been developed in order to avoid the use sodium tetrahydroborate, which is capable of introducing contamination. The sodium tetrahydroborate (NaBH ) - acid reduction technique has been widely used for hydride generation (HG) in atomic spectrometric analyses. However, this technique has certain disadvantages. The NaBH is capable of introducing contamination, is expensive and the aqueous solution is unstable and has to be prepared freshly each working day. In addition, the process is sensitive to interferences from coexisting ions. 

Diborane occupies a special place because all the other boranes can be prepared from it (directly or indirectly) it is also one of the most studied and synthetically useful reagents in the whole of chemistry.B2H6 gas can most conveniently be prepared in small quantities by the reaction of I2 on NaBH4 in diglyme [(MeOCH2CH2)20], or by the reaction of a solid tetrahydroborate with an anhydrous acid ... 

Sodium tetrahydroborate (III) (sodium borohydride ), 1 per cent w/v. Dissolve sodium hydroxide pellets (5.0g) in 300 mL of de-ionised water and cool. Add sodium tetrahydroborate(III) (5.0 g) directly to the sodium hydroxide solution and make up the total volume to 500 mL with de-ionised water. Shake the solution thoroughly and filter through a Whatman No. 541 filter paper. (The resulting solution is stable for at least one week.)... 

Covalent transition metal lanthanide and actinide tetrahydroborate complexes. T. J. Marks and... 

Little information exists on low-T precipitation of borides from solution. Chromium, cobalt and platinum borides are the only ones claimed to have been obtained from aqueous solution ". Ni2B and C02B are precipitated from a solution of nickel or cobalt acetates by adding a solution of sodium tetrahydroborate ". The formation of these borides at RT gives amorphous products. Hence, heat treatment at 300-700°C leads to crystalline NijB as the main constituent, although the average composition of the precipitate corresponds to NijB . 

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