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LIVE LAB 16 • CHEMICAL BONDING
Watch lone-pair donation into a neighbouring vacant orbital, compare 11 back-bonding systems, follow seven shape changes and solve 22 guided challenges.
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COMPLETE CONCEPT GUIDE
Learn back bonding as an orbital-donation interaction, then connect it with bond lengths, bond angles, hybridisation and Lewis-acid trends from foundation to advanced level. The lab compares 11 molecular systems through orbital-flow, hybrid-structure, shape-change and challenge modes.
Back bonding requires a filled donor orbital, a neighbouring vacant acceptor orbital, suitable symmetry for sideways overlap and sufficiently close orbital energies. Donation redistributes electron density and gives the connecting bond partial π character.
Donation between suitably aligned p orbitals is written pπ–pπ. In the classical syllabus description, donation from a p-orbital lone pair toward a suitable acceptor on a third-period atom is commonly represented as pπ–dπ; use the notation required by the question.
In BF₃, fluorine lone-pair density is donated toward the vacant boron 2p orbital. The interaction gives B–F partial multiple-bond character and helps explain the exam order BF₃ < BCl₃ < BBr₃ < BI₃ for Lewis acidity.
Partial π character increases the effective bond order and usually shortens the donor–acceptor bond compared with a related structure where back donation is unavailable. Compare B–F in BF₃ and BF₄⁻ or B–O in H₃BO₃ and B(OH)₄⁻.
Orbital alignment can favour planarity or widen a bond angle. The shape-change mode compares examples including N(SiH₃)₃, SiH₃NCS, H₃BO₃, OCl₂, O(SiH₃)₂, borazine and N(GeH₃)₃.
Both descriptions involve orbital donation, but a coordinate σ bond forms by head-on donation into an acceptor orbital. Back bonding adds a sideways π-type interaction to an existing framework and changes bond character rather than creating an isolated extra bond line.
BACK BONDING FAQ
Back bonding is donation of electron density from a filled orbital on one atom into a suitable vacant orbital on an adjacent atom, producing a π-type interaction and partial multiple-bond character.
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STUDENT DISCUSSION
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A filled donor orbital, a nearby vacant acceptor orbital, compatible symmetry, effective overlap and reasonably matched orbital energies are required. Smaller, well-aligned orbitals generally overlap more effectively.
Electron density moves from a filled 2p orbital on fluorine toward the vacant 2p orbital on boron, so the direction is F(2p) → B(2p).
The standard exam explanation is that 2p–2p donation from fluorine to boron is more effective than the less well-matched donation in BCl₃. This reduces electron deficiency at boron more strongly in BF₃.
The added partial π character raises effective bond order and usually shortens the donor–acceptor bond relative to a comparable bond without back donation.
pπ–pπ notation describes side-on interaction between p orbitals. In classical exam notation, pπ–dπ describes donation from a filled p orbital toward a suitable vacant d-type acceptor orbital on a third-period or heavier atom.
No. A coordinate bond is usually a head-on σ donation in which both shared electrons originate from one atom. Back bonding is an additional sideways π-type donation within an existing bonded framework.
No. It can change hybridisation or bond angle when better orbital alignment requires a different shape, but some molecules keep their basic geometry while their bond order, length or reactivity changes.
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