Tetraborane decomposition

Bond and Pinsky J. Am. Chem. Soc., 92 (32), 1970] have proposed the following mechanism for the decomposition of tetraborane. 

The temperatures of measurable decay range from room temperature for tetraborane to several thousand degrees for water. While the reactions at high temperature are real dissociations to smaller particles, the decomposition occurring at lower temperatures are generally condensations with the simultaneous formation of hydrogen. In the latter case the possibilities for the initial and subsequent elementary steps increase enormously. In these cases it is extremely difficult to obtain the rate of the initial step and even harder to describe the overall reaction in terms of the kinetics of the single steps. 

Interest in the photochemistry of boron compounds dates back as far as 1913 when Alfred Stock investigated the effects of light from a mercury vapor lamp on diboran 6) and on tetraboran 10). In the case of diborane(6) he commented UV light will also decompose B2H6. The volume of a sample in a quartz tube increased by 1/6 after 24 hours exposure to a mercury-arc lamp, and a pale yellow crystalline substance appeared. Stock also observed that B4H q decomposition to B2H is not noticeably influenced by sunlight. 

Gallane, 41 172-173, 196-198 chemical analysis, 41 199, 201 chemical properties, 41 208-209 complexes, 41 175-178 electron diffraction, 41 204-207 2-galla-arachno, tetraborane, 41 216-220 gallaborane, 41 211-216 H NMR spectrum, 41 207-208 IR spectrum, 41 200-202 physical properties, 41 199 search for, 41 173-175 synthesis, 41 198-199 vibrational spectra, 41 200-215 2-Galla-araclino-tetraborane, 41 216-220 "B NMR spectrum, 41 217, 219-220 decomposition of vapor, 41 216-217 electron diffraction, 41 218 structure, 41 217-218 Gallium... 

All of the compounds discussed thus far (except diborane and decaborane) contain only two-center, two-electron bonds. A ample boron hydride containing three types of bonds is tetraborane, B4H 0 (Fig. 16.43). it is formed by the slow decomposition of diborane ... 

There, are at least six definitely characterized boron hydrides, as follows diborane(6), B H tetraborane(lO), B4 H, pentaborane(9) (stable), B3 H pentaborane( 11) (unstable). B5 Hu hexaborane(lO). H10 and decaborane(14). Bio H14. In these names, note that the prefix denotes the number of boron atoms, while the figure in parentheses denotes the number of hydrogen atoms. In addition to these compounds, which are all gases or volatile liquids except decaborane(L4), decomposition of the lower boron hydrides yields colorless or yellow solid boron hydrides, ranging in composition from (BItysty to (BII). This readiness to polymerize is evidence of the reactivity of these borane compounds, which readily form additional products with ammonia, with the amalgams of the active metals, and with many otganic compounds, as well as with CO. 

When kept at room temperature diborane suffers about a ten percent decomposition per year. Tetraborane, unstable pentaborane and hexaborane decompose more rapidly, but stable pentaborane and decaborane, only very slowly below 150°. Diborane is rapidly attacked by cold water, yielding boric acid and hydrogen, but stable pentaborane is completely... 

The reaction of lithium phosphinidodiborane with butyl lithium and borane will give phosphin-ido triborane and phosphinido tetraborane ions (9.70) and (9.71). These compounds are colourless crystals which dissolve in water without decomposition, but are oxidised in air. Potassium phosphinidodiborane, on the other hand, decomposes violently in water to give phosphine and hydrogen. Dry hydrogen chloride reacts with the sodium salt in ether solution to give m-phosphinidiborane (9.72), which is stable only below -20°C. 

Kaufman et al discuss the application of gas chromatography to the analysis of boron hydrides. Using as a column packing Celite coated with paraffin oil (Octoils) or with tricresyl phosphate, they resolved, by the procedure described below, diborane, tetraborane and pentaborane without decomposition on the column. Extensive decomposition occurred on the column, however, in the case of dihydropentaborane,... 

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