The structure of diborane (B2H6) contains:

In diborane (B₂H₆), the Boron atoms are connected to two types of Hydrogens:

• Terminal hydrogens
• Bridging hydrogens

As is clear from the image, the 4 terminal hydrogens are connect to boron by covalent bonds and hence, there are four  2C - 2e^- bonds

The 2 bridging hydrogens though are connected to two boron atoms each making a banana or a 3C - 2e^-