Why BF3 exists when BH3 does not?

Actually BH3 does exist. It is a very common reagent used in the hydroboration of double boonds, and in the reduction of amides to amines. After adding across double bonds the borane can be hydrolysed to the alcohol, effectively completing the addition of water across a double bond. Improtantly this addition is completed in an anti-Markovnikov manner, because in BH3 the H are electronegative, whereas in water they are electropositive.

Because of its instability it exists as a dimer, B2H6, where elextrons are shared between each of the monomers. It is a gas, but is used in chemistry as solutions in THF, pyridine or dimethyl sulfide. In these solutions the lone pair of electrons from the oxyge,, nitrogen or sulfur, are donated to the empty p orbital of boron, stabilising the BH3:solvent complex.

Borane chemistry is a massive field and has enormous use in synthetic organic chemistry, with BH3 being exceptionally useful for a number of transformations and funcional group interconversions.

I have used it many times, and it has been around for well over 50 years. It can be purchased from many sources, including that listed below provided to demonstrate its commercial availability.

If you want more information let me know.

I hope this helps.

Well in BF3 flourine has 3 lone pair of electrons. The high amount of electronegativity of flourine hels stabilize BF3. Moreover we may also find resonance structures for BF3. But in BH3 the stablility cannot be achieved.

The resonating structures are as under!!

F(1),F(3)-B(-)=F+(2) F(2),F(3)-B(-)=F+(1)

F(1),F(2)-B(-)=F+(3)

You'd only have 6 electrons in the outer shell if the electrons were shared in a covalent bond. Not stable. Ionic bond ain't gonna happen because they are both positive ions.

BF3 will get you 8 in the outer shell...nice covalent action.

lester nailed it on this one! Very good answer!