Read Back-Bonding as a Second, Reverse Electron Donation
Back-bonding is electron donation running the OPPOSITE direction to the main bond — a filled p-orbital on a substituent donates into an empty orbital on the central atom, adding partial double-bond character to what would otherwise be a single bond.
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Back-bonding makes BF₃ planar and shortens the B-F bond
Each fluorine's filled p-orbital donates into boron's empty p-orbital (pπ-pπ back-bonding), giving B-F partial double-bond character — this shortens the B-F bond length below a pure single-bond estimate and locks BF₃ into a planar geometry, since sp₂ planarity maximises this p-orbital overlap. Back-bonding therefore changes bond LENGTH, but does NOT change hybridisation or bond angle by itself — those follow from the underlying sp₂ domain count, not from back-bonding directly.
pπ-dπ back-bonding needs an empty d-orbital acceptor
When the central atom is period 3+ (like P, S, Cl, Xe) and a substituent has a lone pair to donate, back-bonding can use the central atom's empty d-orbitals (pπ-dπ) instead of p-orbitals — this is why species like ClO₂⁺ or POCl₃ show multiple-bond character even where a simple Lewis structure would draw only single bonds.
Adduct formation vs back-bonding vs redox — three different electron-movement stories
NH₃ + BF₃ forming an adduct is simple Lewis acid-base donation (NH3's lone pair fills BF3's empty p-orbital) — this changes BOTH atoms' hybridisation (B: sp₂→sp₃, N: stays sp₃) since a new bond forms, and it introduces NEW back-bonding character between the just-bonded B and N. None of this is a redox process, since no oxidation states change — don't confuse adduct/coordinate bond formation with electron transfer.
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- ✓Back-bonding is reverse donation from a substituent's filled orbital into the central atom's empty orbital, adding partial double-bond character.
- ✓Back-bonding shortens bond length and can enforce planarity, but hybridisation/bond angle still follow from the underlying domain count.
- ✓pπ-dπ back-bonding needs an empty d-orbital acceptor, available only for period 3+ central atoms.
- ✓Lewis acid-base adduct formation is electron donation into a new bond, not a redox process — no oxidation states change.
Don't confuse back-bonding (which shortens and strengthens an EXISTING bond via reverse donation) with ordinary Lewis acid-base adduct formation (which creates a NEW bond) — a question can test both in the same molecule, like BF₃ accepting NH3's lone pair while simultaneously losing some of its own F-back-bonding character.
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