Boron fluoride exists as BF_3 but boron hydride doesn't exist as BF_3. Give reason. In which form, does it exist ? Explain its structure.

Boron fluoride exists as BF_3 but boron hydride doesn't exist as BF_3.

1.	Boron fluoride exists as BF_3 but boron hydride doesn't exist as BF_3. Give reason. In which form, does it exist ? Explain its structure.
Answer» SOLUTION :In `BF_3`, lone pair of fluorine gives back support of electrons to boron atom by `ppi-ppi` back bonding. This delocalization of electrons reduces the DEFICIENCY of electrons and THUS reduces Lewis acidic character and increases stability of `BF_3`. In `BH_3`, there is no lone pair of electrons on H atom, therefore, `BH_3` dimerizes to give `B_2H_6`. 4 terminal H atoms and 2 boron atoms lie in one plane and above and below the plane. So there are TWO bridging H-atoms.