Influence of added hydrogen on the kinetics and mechanism of thermal decomposition of tetraborane(10) and of pentaborane(11) in the gas phase

Abstract

The effects of added hydrogen on the kinetics of the first-order thermal decomposition of the two arachno species tetraborane(l0) and pentaborane(11) have been studied in detail by a mass-spectrometric method. In the case of B₄H₁₀, the order and activation energy were unaltered, but the reaction rate was retarded and there was a marked change in product distribution: the percentage yield of B₅H₁₁ remained the same, but B₂H₆ was formed in preference to B₅H₉, B₆H₁₂, B₁₀H₁₄, and involatile solids. These results provide cogent new evidence that B₄H₁₀ decomposes via the single rate-determining step (i), but raise doubts about the validity of subsequent steps in the B₄H₁₀⇌{B₄H₈}+ H₂(i) previously proposed mechanism. In the thermolysis of B₅H₁₁ there was a dramatic change in product distribution, but the order, activation energy, and initial rate of disappearance of B₅H₁₁ were all unaffected by the presence of the added H₂. These results establish for the first time that the so-called ‘equilibrium’(ii) proceeds in the forward direction via the rate-determining B₅H₁₁+ H₂⇌ B₄H₁₀+½B₂H₆(ii) dissociation (iii), followed by the rapid reactions (–i) and (iv). They also imply that in the thermolysis B₅H₁₁→{B₄H₈}+{BH₃}(iii), 2{BH₃}→ B₂H₆(iv) of B₅H₁₁ in the absence of added H₂ the reactive intermediate {B₄H₈} reacts subsequently with itself and is not consumed by reaction with B₅H₁₁.