The number of electrons shared between the two Boron atoms directly in the formation of bonds in diborane molecule
Number of electrons shared between two boron atoms in diborane is zero.
In diborane each B atom undergoes sp3 hybridisation ,it contains two B-H-B bridges . Each bridge bond is formed by overlapping of one "B"sp3 hybrid orbital with one electron, hydrogen s-orbital with one electron and another boron sp3 hybrid orbital without electron .That is the bridge head bond is a electron deficient three centre two electron bond.
The number of atoms involved in bridged bonds in one diborane molecule is four.