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.
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 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.
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.
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.