What Is Back Bonding Mechanism Examples and Applications
Covalent bonds are formed by sharing of electrons. Electron clouds in an atom are arranged in orbitals. The orbitals overlap to form bonds. Sometimes electron pairs from an electron rich element drift back into an empty orbital of comparable energy of the other atom, this donation of electrons back to an atom results in a secondary bonding interaction. This bonding is known as back bonding.
BF3 is a planar covalent compound which displays a pi-back bonding. Lone pair of fluorine is donated to the empty orbital of boron, to form a pi-bonding interaction.
Physical Properties
BF3 is an inorganic covalent compound, it is found in gaseous form. It is colourless, pungent, and toxic and forms white fumes when it comes in presence of moisture.
Table: Properties of BF3
Types of Covalent Bonding
Sigma Bonding: It happens for single bonds where the atomic orbitals overlap head-to-head. It’s the first covalent bond that forms between any atoms.
Pi Bonding: It happens for double and triple bonds. The orbitals participate in the sideways overlap. The second and third covalent bonds are formed in this manner.
Sigma bonds can be formed by hybridised orbital and they are much stronger than pi bonds. The pi bonds are easier to break.
BF3 Molecule
BF3 is known as boron trifluoride. It is a trigonal planar molecule with three fluorine atoms attached to a boron. The fluorine atom occupies the three vertices of the triangle with boron in the middle. The fluorine atoms are singly bonded to boron.
Trigonal Planar Boron Trifluoride
Let’s examine the electronic configuration of boron and fluorine.
Boron, atomic number 5: 1s2 2s2 2p1
Fluorine, atomic number 9: 1s2 2s2 2p5
We can see fluorine is one electron short of its octet, while boron is one electron excess of its octet. Both the atoms first undergo sp2 hybridisation, where the 2s orbital and two 2p orbital hybridise to form three sp2 hybrid orbital. In boron, each three sp2 orbitals contain one electron each bond is directed in three directions in a plane forming a planar triangular shape. The fluorine atom also undergo sp2 hybridisation, the three hybrid orbital contain five electrons, two of the three hybrid orbital contain a pair of electrons each, the third sp2 orbital has one unpaired electron which is extended towards the boron. The sp2 orbital of fluorine and boron containing one electron each overlap head on to form a sigma bond. Three fluorine atom bonds in this manner with the three sp2 orbitals of boron to form the BF3 molecule.
Back Bonding in BF3
In both boron and fluorine, the two 2p orbital and one 2s orbital are engaged in sp2 hybridization, leaving one p orbital free. Boron hosts one unhybridized free p orbital which is empty, while the free p orbital on fluorine contains a pair of electrons. This filles p-orbital over fluorine partially interacts with the empty p-orbital of boron, in a sideway overlap resulting in a pi-bonding interaction. This is the back bonding observed in BF3.
Back Bonding in BF3
Comparing the electronegativity of fluorine and boron, fluorine being highly electronegative pulls the bonding electron towards itself resulting in a partial positive charge over boron and partial negative charge over fluorine, the excess charge is stabilised by the backward pi electron drift from fluorine to boron by the back bonding.
BF3 Chemistry
The BF3 molecule is a perfect trigonal planar molecule with F—B—F angle 120°. The BF bond is shorter than the expected single bond length, this is attributed to the strong pi back bonding. Each B-F bond is polar, but the dipole moment of each B—F bond cancels each other, and as a result, the BF3 molecule is overall non polar. It is categorised as an electron deficient molecule and hence can act as lewis acid.
Some reaction of BF3 demonstrates its ability to form adduct with lewis bases (such as fluorides and ethers)
CsF + BF3 → CsBF4
C2H5—O—C2H5 + BF3 → BF3.O(C2H5)
BF3 reacts with water to form an aquo adduct, which subsequently loses HF to give fluoroboric acid. Hydrolysis of BF3 produces the well-known boric acid [(B(OH)3].
4 BF3 + 3H2O → 3HBF4 + B(OH)3
Uses of BF3
BF3 serves as reagent in organic chemistry, where it acts as lewis acid.
BF3 serves as the building block of all boron compounds.
Used in fumigation.
Used in soldering magnesium.
It is used as a neutron detector.
Key Features
BF3 is an inorganic covalent compound.
BF3 has a trigonal planar structure.
Boron has an empty p-orbital and fluorine has a filled p-orbital.
The p-electron from fluorine drifts into empty p-orbital boron resulting in a pi-bonding interaction, which is known as back bonding.
Interesting Facts
Back bonding is a type of intermolecular lewis acid-base interaction.
BF3 was discovered in 1808 by Joseph Louis Gay-Lussac and Louis Jacques Thenard.
FAQs on Back Bonding Explained in Coordination Chemistry
1. What is back bonding in chemistry?
Back bonding is a type of chemical bonding in which a filled orbital of one atom donates electron density into an empty antibonding orbital of another atom, strengthening the bond between them.
- It usually involves donation from a filled p or d orbital.
- Common in molecules where one atom has a lone pair and the other has a vacant orbital.
- It is also called π back donation or back donation.
2. How does back bonding occur?
Back bonding occurs when a filled orbital of one atom overlaps sideways with an empty π* (antibonding) orbital of another atom, allowing electron density to flow backward.
- Step 1: One atom must have a filled lone pair or filled d orbital.
- Step 2: The other atom must have an empty π* or vacant p/d orbital.
- Step 3: Proper orbital overlap enables sideways (π) interaction.
3. What is pπ–pπ back bonding?
pπ–pπ back bonding is the overlap between a filled p orbital of one atom and an empty p orbital of another atom to form a π bond.
- Common in second-period elements.
- Example: In BF3, fluorine donates lone pair electrons (2p) into the empty 2p orbital of boron.
- This reduces electron deficiency of boron and shortens the B–F bond.
4. What is pπ–dπ back bonding?
pπ–dπ back bonding is the overlap of a filled p orbital of one atom with an empty d orbital of another atom to form a π bond.
- Occurs when one atom has accessible empty d orbitals.
- Common in third-period and transition elements.
- Example: In PCl5, chlorine donates electron density from its p orbital into empty d orbitals of phosphorus.
5. Why is back bonding important in BF3?
Back bonding in BF3 reduces the electron deficiency of boron and strengthens the B–F bonds.
- Boron has only six valence electrons and an empty 2p orbital.
- Fluorine donates lone pair electrons via pπ–pπ back bonding.
- This gives partial double bond character to B–F bonds.
6. What is the difference between back bonding and coordinate bonding?
Back bonding involves π electron donation into an antibonding orbital, whereas coordinate bonding involves σ donation of a lone pair to form a single bond.
- Coordinate bond: Lone pair donated into an empty orbital to form a σ bond (e.g., NH3 → H+ forming NH4+).
- Back bonding: Electron density donated into a π* orbital, strengthening the existing bond.
- Back bonding usually accompanies σ bonding in coordination complexes.
7. How does back bonding affect bond length and bond strength?
Back bonding decreases bond length and increases bond strength by introducing partial double bond character.
- Electron density shared through π overlap increases bonding interaction.
- Bond order effectively increases.
- Shorter bonds correspond to stronger bonds.
8. Can you give an example of back bonding in coordination compounds?
An example of back bonding in coordination chemistry is metal–carbonyl complexes such as Ni(CO)4.
- CO donates a lone pair to Ni via σ bonding.
- Nickel donates electron density from filled d orbitals into the π* orbital of CO.
- This is called dπ–pπ back bonding.
9. Why does back bonding weaken the C–O bond in metal carbonyls?
Back bonding weakens the C–O bond because electron density is transferred into the antibonding π* orbital of CO.
- The π* orbital is antibonding with respect to the C–O bond.
- Electron addition lowers the C–O bond order.
- This results in a longer and weaker C–O bond.
10. What are the conditions required for back bonding?
Back bonding requires a filled donor orbital, an empty acceptor π* orbital, and effective orbital overlap.
- The donor atom must have lone pair electrons or filled d orbitals.
- The acceptor must have low-energy empty π* or vacant p/d orbitals.
- Proper energy match and parallel orbital orientation are essential.
