# How many π bonds are in a molecule of boron trifluoride?

Feb 4, 2014

The quick answer is, “There are no pi bonds in $\text{BF"_3}$, but the answer isn't quite that simple.

#### Explanation:

You can write four Lewis structures for ${\text{BF}}_{3}$.

In one structure, the $\text{B}$ atom has 3 single bonds to the $\text{F}$ atoms and an incomplete octet.

The other three structures have double bonds to the $\text{F}$ atoms. Most texts say that the double bond structures do not occur, because they put a positive charge on the $\text{F}$ atom and result in a separation of charge.

Therefore, ${\text{BF}}_{3}$ has no π bonds.

However, resonance theory says that when you can write more than one Lewis structure for a molecule, the actual structure is a resonance hybrid of them all.

The double bonded structures may be minor contributors, but they still contribute a little bit.

In that sense, the $\text{B-F}$ bonds have some π character.

Furthermore, the length of the $\text{B-F}$ bonds is shorter than expected for single bonds.

This is consistent with some degree of π bonding in ${\text{BF}}_{3}$. 