The structure of diborane (B2H6) contains
(a) four 2c-2e- bonds and four 3c-2e- bonds
(b) two 2c-2e-

Q: Why doesn't diborane just form a B-B single bond like ethane ($C_2H_6$)?

Boron only has 3 valence electrons. If it formed a structure exactly like ethane, it wouldn't have enough electrons to form all the necessary 2-center-2-electron bonds. It is an electron-deficient molecule, which forces it to adopt a different bonding strategy.

Q: What exactly is a $3c-2e^-$ bond?

It's a unique bond where three atoms (in this case, B-H-B) share a single pair of electrons. The electron cloud is delocalized over all three nuclei, forming a curved shape that chemists affectionately call a 'banana bond'.

Q: Are the terminal and bridging B-H bonds the same length?

No! The bridging $3c-2e^-$ bonds are longer and weaker than the terminal $2c-2e^-$ bonds. This is because the electron density in the bridging bond is spread out over three atoms instead of just two.

Q: Is the diborane molecule planar?

No, it is non-planar. The two boron atoms and the four terminal hydrogen atoms lie in the same plane, but the two bridging hydrogen atoms lie above and below this plane, giving it a 3D structure.

Q: What is the hybridization of Boron in diborane?

Each Boron atom in diborane is $sp^3$ hybridized. It uses its four $sp^3$ hybrid orbitals to bond with four hydrogens (two terminal hydrogens and two bridging hydrogens).

Question

The structure of diborane ($	ext{B}_2	ext{H}_6$) contains

Options:

(A) four $2c-2e^-$ bonds and four $3c-2e^-$ bonds

(B) two $2c-2e^-$ bonds and two $3c-3e^-$ bonds

(C) two $2c-2e^-$ bonds and four $3c-2e^-$ bonds

(D) four $2c-2e^-$ bonds and two $3c-2e^-$ bonds

Sigma Percentile · Accepted Answer

## The Fascinating Structure of Diborane: Unpeeling the Banana Bond

Diborane ($B_2H_6$) is one of the most iconic molecules in inorganic chemistry, primarily because it defies the simple rules of covalent bonding we learn early on. Let's take a thrilling journey into its structure and understand why it behaves the way it does.

### The Electron Deficiency Dilemma

Imagine a Boron atom. It sits in Group 13 of the periodic table, meaning it has only 3 valence electrons. Naturally, it wants to form three bonds to share electrons and get closer to a stable octet. It does this by bonding with three hydrogen atoms to form Borane ($BH_3$).

But there's a catch! Even after forming $BH_3$, Boron only has 6 electrons in its valence shell. It is still electron-deficient and highly unstable. To solve this crisis, two $BH_3$ molecules dimerize (join together) to form Diborane ($B_2H_6$). But how do they bond without any extra electrons? This is where the magic happens.

### The Standard Terminal Bonds

In the $B_2H_6$ molecule, we have two central Boron atoms. Each Boron atom forms standard covalent bonds with two 'terminal' Hydrogen atoms.

These are your everyday, normal bonds. Two atoms (one Boron and one Hydrogen) share exactly two electrons. Because the bond exists between two centers and involves two electrons, we scientifically call them **$2c-2e^-$ bonds**.

If you count them up, each Boron has two of these terminal bonds, giving us a total of **four $2c-2e^-$ bonds** in the entire molecule.

### The Magic of Banana Bonds

Now, look closely at the remaining two Hydrogen atoms. They don't just attach to one Boron; they act as a bridge between the two Boron atoms.

Because there aren't enough electrons to form normal bonds, the two Boron atoms and one bridging Hydrogen atom share a single pair of electrons. The electron cloud is delocalized over all three nuclei, creating a curved, banana-like shape.

Since this bond involves three centers (B-H-B) but only two electrons, it is famously known as a **$3c-2e^-$ bond** (or a banana bond). There is one bridging Hydrogen above the plane and one below, giving us exactly **two $3c-2e^-$ bonds**.

### The Final Tally

Putting it all together, the structure of diborane is a beautiful mix of standard and electron-deficient bonding. It contains exactly **four $2c-2e^-$ bonds** and **two $3c-2e^-$ bonds**.

Furthermore, to accommodate this unique geometry, both Boron atoms undergo **$sp^3$ hybridization**. The molecule is strictly **non-planar**; the two Boron atoms and the four terminal Hydrogens lie in one plane, while the two bridging Hydrogens sit perpendicularly above and below this plane.

Understanding this structure is a massive leap in mastering p-block chemistry!

Animated Solution for Chemistry - s and p-Block Elements: The structure of diborane ( $B_{2} H_{6}$ ) contains

Select Answer:

Visualized Solution

\text{Diborane } (\text{B}_2\text{H}_6)

\text{Terminal Bonds}

\text{Counting } 2c-2e^- \text{ Bonds}

\text{Bridging Bonds}

\text{Counting } 3c-2e^- \text{ Bonds}

\text{Final Structure}

\text{Key Takeaways}

The Sigma Insight: Group 13 Elements

The Electron Deficiency Dilemma

The Standard Terminal Bonds

The Magic of Banana Bonds

The Final Tally

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Animated Solution for Chemistry - s and p-Block Elements: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The structure of diborane (B2​H6​) contains