Advanced Topic 214 of 1102 Bonding & Molecular Structure
Three-Center Two-Electron Bond
A bond in which two electrons bind three atoms, as in diborane.
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In-depth explanation of Three-Center Two-Electron Bond
A three-center two-electron bond involves two electrons binding three atoms together. It occurs in electron-deficient molecules that have fewer electron pairs than would be needed for conventional two-center bonds between every adjacent pair. The two electrons occupy a bonding molecular orbital spread over the three atoms. Such bonds are found in boranes, in some carbocations and in certain metal alkyl compounds. They contribute to the unusual structures and reactivity of these species.
Going deeper: Three-Center Two-Electron Bond
Key ideas: Use MO counting: three atomic orbitals give one bonding, one nonbonding and one antibonding MO, and two electrons fill the bonding one, so each link has bond order 0.5. In B2H6 there are 12 valence electrons, 4 terminal B-H bonds use 8, leaving 4 for the two B-H-B bridges. Bridge B-H length is 133 pm and terminal is 119 pm.
Common mistakes and connections: Students draw diborane like ethane with a B-B bond, or assign two electrons to each bridge bond. Another error is forgetting that bonding in bridges is weaker so the bridge hydrogens are the more reactive. Related species include Al2Me6 and the H3+ ion. Links to electron-deficient molecules, boranes, MO theory and incomplete octets.
Visualization of Three-Center Two-Electron Bond
Three orbitals combine into one bonding, one nonbonding and one antibonding MO. Only the bonding MO is filled, with two electrons binding three atoms. The interactive lesson shows this as a live animation or chart.
Worked example and use case
In diborane, B2H6, the two bridging hydrogen atoms are each held by a three-center two-electron bond.
Frequently asked questions about Three-Center Two-Electron Bond
What is Three-Center Two-Electron Bond?
A bond in which two electrons bind three atoms, as in diborane.
Is Three-Center Two-Electron Bond a beginner, intermediate or advanced topic?
Three-Center Two-Electron Bond is a advanced topic in Bonding & Molecular Structure. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Three-Center Two-Electron Bond?
Study these earlier topics in Bonding & Molecular Structure first: Isomerism in Complexes, Hydrogen Molecule Ion, Hypervalency.
What comes after Three-Center Two-Electron Bond?
The next topics in Bonding & Molecular Structure are Banana Bonds and Van der Waals Radius.
Can I practise Three-Center Two-Electron Bond with quizzes?
Yes. The interactive lesson includes a self-check question on Three-Center Two-Electron Bond, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Where Three-Center Two-Electron Bond fits in Bonding & Molecular Structure
Three-Center Two-Electron Bond is topic 99 of 104 in Bonding & Molecular Structure.