Explain why BF3 exists whereas BH3 does not?
BF3 is trigonal planar molecule. Due to pπ - pπ back bonding lone pair of electrons of F is back donated to B atom. This delocalization reduces the deficiency of electrons of boron thereby increasing the stability of BF3 molecule. Thus, due to absence of lone pair of electrons on H atom this compensation does not occur in BH3. In other words electron deficiency of B stays & hence it reduces its electron deficiency as BH3 dimerises to form B2H6.
