In the previous post, I found intriguing the mechanism by which methane (CH4) inverts by transposing two of its hydrogens. Here I take a look at silane, SiH4.
It appears it is a three-stage process! Firstly, silane eliminates molecular hydrogen to form a molecular complex between H2 and SiH2 (DOI: 10.14469/hpc/2290). The barrier (~60 kcal/mol) is very much lower than with methane.
The H2 component of this complex then rotates (DOI: 10.14469/hpc/2289) transposing atoms 1 and 2. The barrier for this process is tiny (~4 kcal/mol).
Finally, the rotated H2/SiH2 complex goes back to silane by the first route, but now with the two hydrogens transposed.
So this inversion is a stepwise process in contrast to methane which was concerted, albeit with “frustrated” elimination of hydrogen. Again a little molecule can show us so much chemistry, in this case also illustrating the avoidance of a Woodward-Hoffmann forbidden cheletropic elimination by desymmetrisation.
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