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 an unusual electron-deficient structure. Unlike ethane (C₂H₆), it lacks sufficient valence electrons for typical two-center, two-electron (2c-2e) bonds between all atoms.\r\n\r\nDiborane's structure includes:\r\n

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• Two boron atoms.
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• Terminal Hydrogens: Four hydrogen atoms are directly bonded to the boron atoms (two per boron). These form standard 2c-2e B-H covalent bonds and are termed terminal hydrogens.
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• Bridging Hydrogens: Two hydrogen atoms bridge the two boron atoms, located above and below the plane of the boron atoms and terminal hydrogens. These form three-center, two-electron (3c-2e) bonds, also known as "banana bonds". These are the bridging hydrogens.
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\r\n\r\nStructure visualization:\r\n(Simplified representation: H_t = Terminal H, H_b = Bridging H)\r\n\r\nHydrogen count:\r\n

\r\n
• Terminal Hydrogens: 4
\r\n
• Bridging Hydrogens: 2
\r\n

\r\nThe question seeks the number of terminal and bridging hydrogens, respectively. The answer is 4 and 2. \r\n