Question

The number of 2 -centre- 2 -electron and 3-centre-2-electron bonds in $B _{2} H _{6}$, respectively, are :

• A. 2 and 4
• B. 2 and 1
• C. 2 and 2
• D. 4 and 2

Answer 1

The Correct Option is D

The molecule $B_{2}H_{6}$ is commonly known as diborane. Its structure is an important topic in inorganic chemistry, especially for understanding electron-deficient compounds and bonding concepts such as 2-centre-2-electron (2c-2e) and 3-centre-2-electron (3c-2e) bonds.

Let's break down the structure of diborane:

1. Diborane consists of two boron (B) atoms and six hydrogen (H) atoms.
2. The traditional Lewis structure model cannot adequately describe all the bonds in diborane because it is electron-deficient.
3. 2-centre-2-electron bonds (2c-2e): These are the traditional covalent bonds where two electrons are shared between two atoms.
4. In $B_{2}H_{6}$, four hydrogen atoms are each bonded directly to a boron atom, forming four 2c-2e bonds (i.e., conventional B-H bonds).
5. 3-centre-2-electron bonds (3c-2e): This is a type of electron-deficient bond involving three atoms sharing two electrons, often referred to as "banana bonds".
6. In diborane, the remaining two hydrogen atoms form bridges between the two boron atoms, creating two 3c-2e bonds. Each bridge bond involves one boron atom from each side and the bridging hydrogen atom in a 3c-2e configuration.

Based on the above structure, the number of 2c-2e and 3c-2e bonds in diborane are as follows:

• 4 2c-2e B-H bonds (terminal hydrogen atoms bonded to boron).
• 2 3c-2e bonds (involving the bridging hydrogen atoms).

This matches the correct answer: 4 and 2.