Question

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

Answer 1

BF₃ exists as a monomer due to pπ−pπback bonding. Fluorine transfers two electrons to vacant 2p-orbital of boron. The delocalisation reduces the deficiency of electrons on boron thereby increasing the stability of BF₃ molecule. Due to absence of lone pair of electrons on H, the back bonding does not occur in BH₃. In other words, electron deficiency of boron remains and BH₃ does not exist. To reduce electron deficiency BH₃ dimerises to form B₂H₆. Hence, boron hydride exists in dimeric form and known as diborane. Structure of diborane is already given in exercise 8.19(NCERT).