From this website, I only know BH3 exist as dimer forms.
But I still don't know why BH3 does not exist as monmeric form!!!
Also electron configuration of boron is (2,3), that is, 1s22s22p1
excited state is 1s22s12p2.
Then your question seems to increase my misunderstanding!
Ok, if we consider the hypothetical structure of BH3
, we expect sp2 hybridisation. So the 6 bonding electrons (3 from B and 1 from each H) would be in 3 sp2 orbitals and there would be an empty p orbital (just like a carbocation, but it wouldn't be charged). The Hs would be in a trigonal planar geometry around the B.
Right, problem is, this molecule is very electron deficient and really wants some more electrons to put in that empty p orbital. Next problem, where the hell is it going to get them from? another molecule of BH3
! so the two molecules bond together and form the dimer because they are both so greedy for electrons!
Now, to the question at hand. Why doesn't BF3
do the same thing? Well, the central B still has an empty p orbital as before, but there is a closer source of electrons to put in it than another molecule.... where do you think that source is? Hint: See if you can think of any resonance structures for BF3
It is easier to approach this question from the "why doesn't BF3
dimerise" direction than the "Why does BH3
dimerise" direction. If you see what I mean...