Diborane is an electron deficient molecule. The two boron atoms and the four terminal hydrogen atoms of the molecule are all in the same plane. These four terminal B -H bonds are regular 2-centered- 2 electron bonds.
The bridging hydrogen atoms lie above and below this plane. The two bridges B-H-B bonds are unusual three centered two electron bonds. The boron atoms in diborane undergo sp³ hybridisation. The overlapping of a vacant sp³ hybrid orbital of one boron atom and sp³ hybrid orbital of another boron atom containing one electron with the pure s -orbital of bridging hydrogen containing one electron results in the "banana bond" .Similarly other banana bond is formed on other side .
Thus the two banana bonds formed lie above and below the plane of the boron atoms. The remaining two sp³ hybrid orbitals of each boron atom overlap with terminal hydrogen atoms to form normal sigma bonds, results in the formation of four terminal hydrogen bonds.