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BRIDGE BONDING · ADVANCED CONCEPT

B₂H₆
Diborane banana bonding

Diborane is a central example of electron-deficient multicentre bonding. Its four terminal B–H bonds are ordinary two-centre bonds, while each B–H–B bridge spreads one electron pair across three nuclei.

3c–2e banana bondChemical BondingInteractive 3D lab
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STRUCTURE REASONING

How B₂H₆ bridge bonding works.

Two bridging hydrogen atoms complete the electron-deficient B₂H₆ framework without creating an ordinary B–B bond. In the classical localized-orbital picture, each bridge aligns a hydrogen 1s orbital between two boron-directed orbitals and forms one curved three-centre two-electron region—the banana bond.

BOND COUNT

2 × 3c–2e bridges + 4 × terminal 2c–2e B–H bonds

TERMINAL VS BRIDGE

Bridge B–H contacts are longer and have lower bond order than terminal B–H bonds.

ADVANCED CONCEPT CHECKLIST

Facts to read from the 3D model.

  1. There are two simultaneous B–H–B bridge units, and each unit contains only one electron pair.
  2. Six atoms—two B and four terminal H—can be placed in one plane; the two bridge H atoms are perpendicular to it.
  3. Terminal B–H is about 119 pm, while bridge B–H contacts are about 131–134 pm in the model data.
  4. The bridge-side H–B–H angle is about 97°, compared with about 122° between terminal hydrogens.

SEARCHED QUESTIONS

B₂H₆ bridge-bonding FAQ.

Why is the B2H6 bond called a banana bond?

The B–H–B electron density follows a curved path around the three nuclei, so the three-centre two-electron bridge is commonly called a banana bond.

How many banana bonds are present in diborane?

Diborane contains two B–H–B banana bonds. Each is one 3c–2e unit; the four remaining B–H bonds are terminal 2c–2e bonds.

Does B2H6 contain a normal B–B bond?

No ordinary localized B–B bond is required in this model. The two hydrogen bridges supply multicentre stabilization to the electron-deficient framework.

MOLECULE-BY-MOLECULE GUIDES

Study every bridge-bonding system.

Open a focused structure guide, then use the shared 3D lab to rotate the complete molecule and compare bridge formation.