Count the valence electrons of diborane, B₂H₆.

Step 1. Count. $2 \times 3 + 6 \times 1 = 12$.

Step 2. Ethane-like. Seven bonds of two electrons each need $14$, two more than there are.

Step 3. Bridges. Four terminal bonds use $8$, so $4$ are left: $2$ for each bridge, spread over three atoms.