Boron
Unusual structure and bonding in the boranes, in which hydrogens act as bridges between boron and the B-H-B arrangement shares two electrons between three atoms (called three center bonding)
Because of their electron deficiencies, boranes are highly reactive
Diborane is very reactive:
B2H6(g) + 3O2(g) ? B2O3(s) + 3H2O(g) ?H = -2030 kJ