The number of three centred, 2 electron bonds in diborane is

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.