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.