Tetraboranes

The tetra alkyl amm onium salts of [B Hg] , formed by ion-exchange reactions, have proven to be useful synthetic reagents because of their thermal and air stabihties. The stmcture of the [B Hg] ion has been determined by an x-ray study (66) and shown to have the 2013 styx stmcture, C2 symmetry. Mechanisms for the formation of this ion have been proposed (67). Tetraborane(lO) can be easily obtained from salts of [B Hg] (eq. 9). 

Using the dodecahydropentaboratefl -) ion in reaction with (Ph jPljCuCl results in a change in the borane shape, leading to a tetraborane derivative ... 

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

The reported ignition of diborane and tetraborane(lO) in contact with air or oxygen is due to the presence of traces of silicon hydrides [1]. The lower members of the latter class ignite or explode in air or oxygen, especially at reduced pressure [2], Phosphine is also sensitive [3],... 

Pearson and Edwards98 report that the thermal conversion of tetraborane without diborane being present is also first order, but is about one order of magnitude slower. The experimental energies of activation for both reactions, however, have been found to be almost the same. 

Mass spectrometric investigation of the pyrolysis of tetraborane by Stafford et al." gave evidence for the intermediate B4H8. As in similar work under their experimental conditions the influence of heterogeneous catalysis might be dominant. 

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. 

Phosphacarba- f6>-pentaborane [2-/Bu-l,2-PCB3Fl5] 25 was synthesized by gas-phase reaction of phosphaalkyne with tetraborane(lO) (see Figure 11). In the 31P NMR spectrum a signal at —501 ppm was detected for the apical phosphorus atom and confirmed by quantum mechanical calculations.120... 

Most of these reactions are reversible. Thus, heating tetraborane(lO) in the presence of a large excess of hydrogen reforms diborane. Pentaborane(ll) is thermally... 

It is most likely that unstable B3H7 is generated in a first step. Its dimerization leads to unstable B6Hi4 [32] which decomposes to tetraborane(lO) and non-characterized additional boron compounds, denoted in Eq. (26) as B2H4. ... 

---