Tau bonds are formed by overlapping of

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.