Back Bonding: Concept & Effects
Back Bonding is a special type of coordinate bond (dative bond) formed sideways between two atoms already bonded by a sigma bond. It occurs when one atom has a Lone Pair of electrons and the adjacent atom has a Vacant Orbital.
1. Conditions for Back Bonding
- Donor Atom: Must have a lone pair of electrons.
- Acceptor Atom: Must have a vacant orbital (p-orbital or d-orbital).
- Size Factor: The participating atoms must be of comparable size (typically 2nd or 3rd period). Effective overlap decreases significantly with larger atoms (e.g., $2p-4p$ overlap is negligible).
2. Types of Back Bonding
A. $p\pi - p\pi$ Back Bonding
Occurs when both atoms belong to the 2nd Period. The donor uses a filled 2p orbital and the acceptor uses an empty 2p orbital.
Classic Example: Boron Trifluoride ($BF_3$)
Boron has an empty $2p$ orbital. Fluorine has filled $2p$ orbitals.
Fluorine donates electron density to Boron ($F \to B$).
Consequence: The B-F bond acquires partial double bond character. The Lewis acidity of $BF_3$ decreases.
B. $p\pi - d\pi$ Back Bonding
Occurs when one atom is from the 2nd period (Donor) and the other is from the 3rd Period (Acceptor). The acceptor uses an empty 3d orbital.
Example: Trisilylamine $(SiH_3)_3N$
Nitrogen has a lone pair in $2p$. Silicon has empty $3d$ orbitals. Nitrogen donates electron density to Silicon ($N \to Si$).
3. Effects of Back Bonding
Back bonding significantly alters the physical and chemical properties of molecules.
1. Bond Length
Back bonding introduces partial double bond character, making the bond length shorter than expected for a single bond.
Example: B-F bond in $BF_3$ is shorter than in $BF_4^-$.
2. Bond Angle & Hybridization
Case Study: Trimethylamine vs Trisilylamine
- $(CH_3)_3N$: No back bonding. N is $sp^3$ hybridized. Shape: Pyramidal. Bond Angle $\approx 108^\circ$.
- $(SiH_3)_3N$: $p\pi-d\pi$ back bonding exists ($N \to Si$). The lone pair on N is delocalized into empty d-orbitals of Si. N becomes $sp^2$ hybridized. Shape: Trigonal Planar. Bond Angle = $120^\circ$.
3. Lewis Basicity
If the lone pair is involved in back bonding, it is less available for donation to external acids.
Result: Basicity decreases.
$(SiH_3)_3N$ is a much weaker base than $(CH_3)_3N$.
4. Lewis Acidity
Back bonding satisfies the electron deficiency of the central atom.
Result: Lewis Acidity decreases.
Acidity Order of Boron Halides: $BF_3 < BCl_3 < BBr_3 < BI_3$.
Reason: Back bonding is strongest in $BF_3$ ($2p-2p$ overlap) and weakest in $BI_3$ ($2p-5p$ overlap is ineffective). Hence $BI_3$ is the most electron-deficient (Strongest Acid).
4. Summary of Key Molecules
| Molecule | Type | Effect |
|---|---|---|
| $BF_3$ | $2p\pi-2p\pi$ | Low acidity, Short bond |
| $N(SiH_3)_3$ | $2p\pi-3d\pi$ | Planar, Weak base |
| $O(SiH_3)_2$ | $2p\pi-3d\pi$ | Wide bond angle ($144^\circ$) |
| $CCl_2$ (Singlet) | $3p\pi-2p\pi$ | Stabilizes carbene |
Back Bonding Quiz
Test your concepts on Orbital Overlap. 10 MCQs with explanations.
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