Decomposition tetrahydroborates

A large volume of alkaline tetrahydroborate solution spontaneously heated and decomposed, liberating large volumes of hydrogen which burst the container. Decomposition is rapid when pH is below 10.5 [1], A more detailed account of the investigation was published [2]. Dry mixtures with sodium hydroxide containing 15-40% of tetrahydroborate liberate hydrogen explosively at 230-270°C [3],... 

Contact of a drop of glycerol with a flake of sodium tetrahydroborate leads to ignition, owing to thermal decomposition of the latter at above 200°. Other glycols and methanol also react exothermally, but do not ignite. 

The second processing step consisted of salt decomposition with the subsequent reduction to pure metal. The method of chemical deposition of metal salts from the water salt solution with the subsequent reduction to pure metal by liquid phase reducer has been applied to prepare graphite-tin CMs. In this case tin chloride was used for impregnation and potassium tetrahydroborate was used as liquid phase reducer. 

Another, quite versatile, route to hydropolyborates is based on the thermal decomposition of tetraorganylammonium tetrahydroborates, R4N[BH4]. Here, the products formed depend on temperature and time. For each hydropolyborate reaction conditions have to be optimized. 

In contrast to the interstitial hydrides, where the metal lattice hosts the hydrogen atoms on interstitial sites, the desorption of the hydrogen from the complex hydride leads to a complete decomposition of the complex hydride and a mixture of at least two phases is formed. For alkali metal tetrahydroborates and tetrahydroaluminates, the decomposition reaction is described according to the following equation ... 

For alkaline earth metal tetrahydroborates and tetrahydroaluminates, the decomposition reaction is described according to the following equations ... 

The physical and chemical properties of the tetrahydroborates show more contrasts than the salts of nearly any other anion. The alkali metal salts are the most stable. In dry air, NaBH4 is stable at 300°C and in vacuo to 400°C with only partial decomposition. In contrast, several tetrahydroborates, including the titanium, thallium, gallium, copper, and silver salts, are unstable at or slightly above ambient temperatures. The chemical and physical properties of the tetrahydroborates are closely related to molecular structure. Sodium tetrahydroborate, which is typical of the alkali metal tetrahydroborates except for the lithium salt, has a face-centered cubic (fee) crystal lattice which is essentially ionic and contains the tetrahedral [BHJ- anion. The tetrahydroborates of the polyvalent metals are in many cases the most volatile derivatives of these metals known. Aluminum tris(tetrahydroborate)... 

Styrene derivatives can be selectively converted to the corresponding benzyl alcohols by molecular oxygen in the presence of bis(dimethylglyoximato)chloro(pyridine)cobalt(III) and sodium tetrahydroborate (equation 242).559 A likely mechanism for this reaction involves insertion of the alkene into the cobalt-hydride bond, followed by 02 insertion into the cobalt-carbon bond, as in equation (11), and decomposition of the peroxide adduct (168) to the ketone, which is reduced to alcohol by NaBH4 (equation 243). 

D Ulivo, A. 2004. Chemical vapor generation by tetrahydroborate (III) and other borane complexes in aqueous media—A critical discussion of fundamental processes and mechanisms involved in reagent decomposition and hydride formation. Spectrochim. Acta B 59 793-825. 

ACGIH TLV (Proposed TWA 250 ppm.) SAFETY PROFILE A simple asphyxiant. Very dangerous fire hazard when exposed to heat, flame, or oxidizers. To fight fire, stop flow of gas. Moderately explosive when exposed to flame. Mixtures with aluminum tetrahydroborate explode after an induction period. When heated to decomposition it emits acrid smoke and fumes. 

A strong acid. Mixtures with nitromethane are explosive. Reacts with chlorides + stainless steel to form explosive hydrogen gas. Potentially violent reaction with sodium tetrahydroborate. Dangerous when heated to decomposition it emits toxic fumes of... 

The tetrahydroborates of Ga are well established, although Ga(BHP3 is unstable, and only its decomposition products are isolable ... 

A mechanistic study of the role of the lanthanide cations suggests that they catalyze decomposition of borohydride by the hydroxylic solvent to afford alkoxyborohydrides, which may responsible for the observed regioselectivity. The stereoselectivity of the process is also modified by the presence of Ln " ions, in that axial attack of cyclohexenone systems is enhanced. a,p-Unsaturated aldehydes are regio-selectively reduced to allylic alcohols by bis(triphenylphosphine)copper(I) tetrahydroborate in the presence of Lewis acid catalyst. ... 

Zirconium and hafnium diboride have been studied less extensively. Zirconium diboride potentially is useful as a coating for solar absorbers [242]. These compositions can be prepared either by hydrogen co-reduction of the metal and boron halides [242], or by the decomposition of the metal tetrakis(tetrahydroborates) M(BH4)4 [241]. 

Fig. 6.22 Decomposition temperature for metal tetrahydroalanates and metal tetrahydroborates versus the Pauling electronegativity of the metal atom, which forms the cation. The horizontal line is at 300 K. A...

The stability as an enthalpy difference between the complex hydride and an intermediate product in the decomposition reaction M[BH4] MH + B. AH tetrahydroborates where the MH exhibits a heat of formation between the complex hydride and its elements go through the above-mentioned intermediate product in the thermal desorption of hydrogen. 

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