Q.25 The total number of 3-centre-2-electron bonds in B4H10 is ______ (in integer).
The total number of 3-centre-2-electron (3c-2e) bonds in B₄H₁₀ is 4.
B₄H₁₀, known as tetraborane(10), features a butterfly-shaped arachno-borane structure with four boron atoms and ten hydrogen atoms. This electron-deficient molecule relies on multicenter bonding, specifically four B-H-B bridges that each represent a 3c-2e bond.
Structure of B₄H₁₀
Tetraborane adopts a non-planar C₂ᵥ-symmetric geometry where two boron atoms form wingtips connected by a central B-B bond, and the other two occupy bridgehead positions. Four terminal B-H bonds on the wingtip borons form two BH₂ groups, while six additional B-H bonds include four bridging hydrogens (B₁-H-B₃, B₃-H-B₂, B₂-H-B₄, B₄-H-B₁). One direct 2c-2e B-B bond links B₁-B₂.
STYX Code Analysis
The STYX code for B₄H₁₀ is 4012, where:
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S=4: Number of 3c-2e B-H-B bonds (the four bridges).
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T=0: Number of 3c-2e B-B-B bonds (none present).
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Y=1: Number of 2c-2e B-B bonds.
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X=2: Number of terminal BH₂ groups.
This code satisfies electron counting: 4 boron atoms contribute 12 valence electrons, 10 hydrogens contribute 10, totaling 22 electrons. The bonds require 4×2 (bridges) + 7×2 (terminal B-H and B-B) = 22 electrons.
Bond Types Breakdown
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Four 3c-2e B-H-B bonds: Each involves two boron sp³ hybrid orbitals (one singly occupied, one empty) and one H 1s orbital sharing two electrons, forming banana-shaped bonds.
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Six 2c-2e terminal B-H bonds: Standard covalent bonds on all borons.
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One 2c-2e B-B bond: Direct overlap between boron hybrid orbitals.
No other 3c-2e bonds exist, confirming the total as 4. Common errors include confusing with B₂H₆ (2 bridges) or assuming B-B-B units (T=0 rules this out).
Introduction
In boron chemistry, understanding 3-centre-2-electron bonds in B4H10 is crucial for CSIR NET aspirants tackling electron-deficient boranes. Tetraborane(10) (B₄H₁₀) features exactly four 3c-2e bonds, as confirmed by its STYX code 4012 and butterfly structure. This guide explains the bonding, electron count, and why this integer answer (4) appears in competitive exams.
What Are 3-Centre-2-Electron Bonds?
3c-2e bonds occur in electron-deficient compounds like boranes, where three atoms (typically two B and one H) share two electrons in a delocalized orbital. Unlike 2c-2e bonds, they stabilize structures without enough electrons for all conventional pairs. In B4H10, these manifest as B-H-B bridges.
Molecular Structure of B4H10
B₄H₁₀ exhibits arachno geometry per Wade’s rules (2n+4=10 skeletal electrons for n=4). Key features:
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Butterfly shape with C₂ᵥ symmetry.
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Four bridging H atoms forming B-H-B units.
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Two BH₂ terminal groups, four other terminal B-H, one B-B bond.
Visualize: Borons 1-2 connected directly, bridged to 3-4 via hydrogens.
STYX Rules for B4H10
STYX quantifies borane bonding:
| Parameter | Value | Description |
|---|---|---|
| S | 4 | 3c-2e B-H-B bonds |
| T | 0 | 3c-2e B-B-B bonds |
| Y | 1 | 2c-2e B-B bonds |
| X | 2 | BH₂ groups |
Total 3c-2e bonds = S + T = 4 + 0 = 4.
Electron Counting Verification
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Valence electrons: 4B×3 + 10H×1 = 22.
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Bonds: 4×(3c-2e) + 7×(2c-2e) = 8 + 14 = 22 electrons.
Perfect match—no extra multicenter bonds needed.
Exam Relevance for CSIR NET
Questions like “The total number of 3-centre-2-electron bonds in B4H10 is ______” test STYX application. Differentiate from diborane (B₂H₆: STYX ₂₀₂₀, 2 bridges).
