Question

The number of terminal and bridging hydrogens in B$_2$H$_6$ are respectively

• A. 4 and 2
• B. 2 and 4
• C. 2 and 2
• D. 4 and 4

Hint:

Using a simple frame or just bolding for the box
Key Points:
Diborane (B$_2$H$_6$) is an electron-deficient molecule.
It contains two types of hydrogen atoms: terminal and bridging.
Terminal B-H bonds are normal 2-center, 2-electron bonds (4 total).
Bridging B-H-B bonds are 3-center, 2-electron bonds (2 total).
Total hydrogens: 4 (terminal) + 2 (bridging) = 6.

Answer 1

The Correct Option is A

Diborane (B₂H₆) has a unique structure that is electron-deficient. It does not have enough valence electrons to form conventional two-center, two-electron (2c-2e) bonds for all atom connections like ethane (C₂H₆). The structure of diborane features:

• Two boron atoms.
• Terminal Hydrogens: Four hydrogen atoms are bonded directly to the boron atoms (two hydrogens per boron). These lie roughly in the same plane as the boron atoms and form standard 2c-2e covalent B-H bonds. These are called terminal hydrogens.
• Bridging Hydrogens: Two hydrogen atoms are located above and below the plane formed by the boron atoms and the terminal hydrogens. Each of these hydrogens forms a bridge between the two boron atoms. These bonds are called three-center, two-electron (3c-2e) bonds or "banana bonds". These are the bridging hydrogens.

Visualizing the structure: (Simplified representation where H_t = Terminal H, H_b = Bridging H) Counting the hydrogens:

• Number of Terminal Hydrogens = 4
• Number of Bridging Hydrogens = 2

The question asks for the number of terminal and bridging hydrogens, respectively. This corresponds to 4 and 2.