Question
Boron fluoride exists as $\mathrm{BF}_3$ but boron hydride doesn't exist as $\mathrm{BH}_3$. Give reason. In which form does it exist? Explain its structure.
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Answer
$\mathrm{BF}_3$ exists as a monomer due to $\mathrm{p} \pi$-prtback bonding. Fluorine transfers two electrons to vacant 2 p -orbital of boron. The delocalisation reduces the deficiency of electrons on boron thereby increasing the stability of $\mathrm{BF}_3$ molecule. Due to absenceof lone pair of electrons on H , the back bonding does not occur in $\mathrm{BH}_3$. In other words, electron deficiency of boron remains and $\mathrm{BH}_3$ does not exist. To reduce electron deficiency $\mathrm{BH}_3$ dimerises to form $\mathrm{B}_2 \mathrm{H}_6$. Hence, boron hydride exists in dimeric form and known as diborane.
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