Kinetics, tetrahydroborates

Earlier kinetic studies on the acid hydrolysis of the tetrahydroborate ion (BH4 ) in moist acetonitrile were interpreted in terms of the accumulation of an intermediate complex of acetic acid and the tetrahydroborate anion. However, nmr measurements and kinetic isotope effects now show that while this intermediate is formed, and allows isotope exchange between BH4 and CH3COOD, it does not accumulate. The rate-determining step involves a second intermediate CHaCOOBHa", so that the earlier study on the hydrolysis of tetrahydroborate actually related to the hydrolysis of this species. It is suggested that this occurs... 

To aid the synthesis of hitherto unknown tetrahydroborates, and in the hope of being able to improve reversibility and kinetics, insight into the atomic structures of the tetrahydroborates is needed. Due to the small cross-section for hydrogen in x-ray diffraction and the high absorption of neutrons by boron in neutron diffraction, it is difficult to characterize the... 

Sodium borohydride, or correctly sodium tetrahydroborate, is a white crystalline solid that is stable in dry air to 300°C. It is highly soluble in water and slowly decomposes until the solution becomes alkaline. Decomposition is more rapid in acidic solution, but NaBH4 is kinetically stable in alkaline solutions when the pH is above about 12.9. For this reason, 2%-3% sodium hydroxide is typically added to NaBH4 solutions to provide stability. Although kinetically stable, aqueous solutions of NaBH4 are thermodynamically unstable, and decomposition can be catalyzed by a number of transition metals. 

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