A kinetic study of the gas-phase thermolysis of pentaborane(11)

Abstract

The kinetics of thermal decomposition of pentaborane(11) have been investigated by a mass-spectrometric technique in the pressure range 1.75–10.50 mmHg and temperature range 40–150 °C. In conditioned Pyrex vessels the reaction was shown to occur by a homogeneous gas-phase process according to the first-order initial-rate law –d[B₅H₁₁]/dt= 1.3 × 10⁷ exp(–72 600/RT)[B₅H₁₁]. The main volatile products are H₂ and B₂H₆, the latter appearing at the rate of ca. 0.5 mol per mol of B₅H₁₁ consumed. Pentaborane(9) is also produced, at less than half the rate of B₂H₆, together with even smaller amounts of hexaboranes and B₁₀H₁₄, and traces of B₄H₁₀; some 40–45% of the boron is converted into involatile solid hydride BH_x, where x varies from ca. 2.0 at 40 °C to ca. 1.1. at 150 °C. No obvious dependence on temperature was detected in the overall distribution of boron between volatiles and solid, but the production of B₅H₉ was favoured at higher temperatures. Mechanistic implications of these results are discussed.