Boron fluoride exists as BF₃ but boron hydride doesn't exist as BH₃. Give reason. In which form does it exist? Explain its structure.

<p>This is a Long Answers Type Questions as classified in NCERT Exemplar</p><p>In BF₃, due to n−pπ back bonding between the vacant p-orbital of boron and filled p-orbital of fluorine. This Π− pπ back bonding is absent in case of hydrogen as it is a single electron element.</p><p>Two BH₃ molecules dimerise to form diborane.</p><p>In B₂H₆ There are two types of hydrogens present.</p><p><strong> (I) </strong>Four hydrogens that are terminally bonded to each of two boron atoms.</p><p><strong> (II) </strong>Two hydrogens that are bonded to both boron atoms forming a bridge in between.</p><p>The four terminal hydrogen atoms and two boron atoms lie in the same plane while bridging hydrogen lies in a plane perpendicular to them.</p><p>Two hydrogens forming a bridge in B₂H₆ are peculiar in bonding and can be termed as 3 -centered-2-electron bond or banana bond. 1sorbital&nbsp; of each hydrogen overlaps with the hybrid orbital of one of the boron then delocalising the 2e−over three atoms making 3 -centered- 2 -electron bond.</p>

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