Possible mechanisms of the reaction between tetrahydroborate and hydrogen ions: a permutational analysis

Abstract

A detailed permutational analysis of the reaction sequence [BH₄]^–+ H⁺→ BH₅→ BH₃+ H₂ leads to the suggestion that the reaction, in aqueous solution, could proceed via a rigid C_s intermediate BH₅ having a kind of H₂ subunit as potentially evolving molecular hydrogen, in agreement with previous quantum-mechanical calculations. On the other hand, the anhydrous reaction Na[BH₄]+ H₂SO₄ could proceed via a trigonal-bipyramidal intermediate with edge attack of the [BH₄]^– tetrahedral structure, followed by axial–equatorial loss from the trigonal bipyramid. Finally, it is likely that the anhydrous reaction Na[BH₄]+ HF proceeds via a non-rigid BH₅ intermediate of undetermined structure.