Boron fluoride exists as BF3 but boron hydride doesn’t exist as BH3. Give a reason. In which form does it exist? Explain its structure.
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Solution
BF3 and BH3 existence
BF3 is an electron deficient species, it tries to complete the octet of Boron atom by forming pπ-pπ back bonding with fluorine.
The boron atom in BF3 is sp2 hybridized with a vacant 2p orbital, while the F atom contains three lone pairs.
These lone pairs on F can overlap with the empty 2p orbital on B, thereby forming a coordinate bond. This donation of lone pair of electrons by fluorine is called back bonding. It reduces the deficiency of electrons on boron thereby increasing the stability of BF3 molecule.
Dimer of BF3
In BF3, B is electron deficient and there is no lone pair on the H atom to form pπ−pπ back bonding.
Therefore, to achieve stability it dimerises to form diborane B2H6.
Structure of diborane:
The four terminal hydrogen atoms and the two boron atoms lie in one plane.
Above and below this plane, there are two bridging hydrogen atoms.
The four terminal B-H bonds are regular two centre-two electron bonds while the two bridge (B-H-B) bonds are different and can be described in terms of three centre–two electron bonds. The 3-Centre-2-electron bridge bonds are also referred to as banana bonds.