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

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.

Two BH₃ molecules dimerise to form diborane.

In B₂H₆ There are two types of hydrogens present.

(i) Four hydrogens that are terminally bonded to each of two boron atoms.

(ii) Two hydrogens that are bonded to both boron atoms forming a bridge in between.

The four-terminal hydrogen atoms and two boron atoms lie in the same plane while bridging hydrogen lies in a plane perpendicular to them.

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 of each hydrogen overlaps with the hybrid orbital of one of the boron then delocalising the 2e^– over three atoms making 3-centred- 2 -electron bond.