Ncert Exercises & Solutions

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

Due to

pπ - pπ

back bonding, the lone pair of electrons of F is donated to the B-atom. This delocalisation reduces the deficiency of electrons on B thereby increasing the stability of BF₃ molecule.

Due to absence of lone pair of electrons on H-atom, this compensation does not occur in BH₃. In other words, electron deficiency of B stays and hence to reduce its electron deficiency, BH₃ dimerises to form B₂H₆.

In B₂H₆, four terminal hydrogen atoms and two boron atoms lie in one plane. Above and below this plane there are two bridging H-atoms. The four terminal B-H bonds are regular while the two bridge (B-H-B) bonds are three centre- two electron bonds.

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