Banana Bond: The 3-Center-2-Electron Bond
The Banana Bond (also known as a bent bond or bridge bond) is a specific type of chemical bond found in electron-deficient molecules. The most classic example is Diborane ($B_2H_6$). Unlike normal covalent bonds where two atoms share two electrons (2c-2e), a banana bond involves three atoms sharing two electrons (3c-2e).
1. The Problem with Diborane ($B_2H_6$)
Borane ($BH_3$) is unstable because Boron has an incomplete octet (only 6 valence electrons). It dimerizes to form Diborane ($B_2H_6$).
Electron Counting Anomaly
Total valence electrons in $B_2H_6$:
Boron ($3e^-$) $\times$ 2 = 6 electrons
Hydrogen ($1e^-$) $\times$ 6 = 6 electrons
Total = 12 electrons.
A standard ethane-like structure ($H_3B-BH_3$) would require 7 covalent bonds (14 electrons). Since only 12 are available, Diborane is Electron Deficient.
2. Structure of Diborane
The structure contains two types of hydrogen atoms:
- 4 Terminal Hydrogens ($H_t$): Form normal covalent bonds with Boron.
- 2 Bridging Hydrogens ($H_b$): Lie between the two Boron atoms.
Geometry
The two Boron atoms and the four terminal hydrogens lie in the same plane. The two bridging hydrogens lie above and below this plane, perpendicular to it.
3. The 3-Center-2-Electron (3c-2e) Bond
This is the "Banana Bond".
Orbital Overlap Mechanism
- Each Boron atom is $sp^3$ hybridized.
- Of the four $sp^3$ hybrid orbitals on Boron:
- Two form normal bonds with terminal hydrogens ($B-H_t$).
- One contains an electron.
- One is empty.
- Formation of the Bridge:
One $sp^3$ orbital from Boron 1 (with 1 electron), the $1s$ orbital of Hydrogen (with 1 electron), and the empty $sp^3$ orbital of Boron 2 overlap.
3 Atoms involved, 2 Electrons shared.
This delocalization creates a bent, banana-shaped electron cloud above and below the B-B axis. Hence, it is called a Banana Bond.
4. Bond Characteristics
| Parameter | Terminal Bond ($B-H_t$) | Bridge Bond ($B-H_b-B$) |
|---|---|---|
| Bond Type | 2-center-2-electron (2c-2e) | 3-center-2-electron (3c-2e) |
| Strength | Stronger | Weaker |
| Bond Length | Shorter (119 pm) | Longer (134 pm) |
| Reactivity | Less Reactive | More Reactive (Cleaves easily) |
5. Other Examples
- Polymeric Beryllium Hydride $(BeH_2)_n$: Also contains 3c-2e bonds ($Be-H-Be$) in its polymeric solid structure.
- Trimethylaluminum dimer $Al_2(CH_3)_6$: Contains methyl bridges ($Al-CH_3-Al$) which are 3c-2e bonds.
Don't Confuse with Coordinate Bridge!
In $Al_2Cl_6$, the bridge is formed by Chlorine donating a lone pair. This is a 3-center-4-electron (3c-4e) coordinate bond, NOT a banana bond. Banana bonds specifically refer to electron-deficient 3c-2e bonding.
6. Properties of Diborane
- It acts as a strong Lewis Acid because it is electron deficient.
- It reacts with Lewis bases (like amines) to undergo symmetrical or unsymmetrical cleavage of the banana bonds.
Banana Bond Quiz
Test your concepts on Chemical Bonding. 10 MCQs with explanations.
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