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

1 Answer
Feb 4, 2014

Answer:

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

Explanation:

You can write four Lewis structures for #"BF"_3#.

In one structure, the #"B"# atom has 3 single bonds to the #"F"# atoms and an incomplete octet.

The other three structures have double bonds to the #"F"# atoms.

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Most texts say that the double bond structures do not occur, because they put a positive charge on the #"F"# atom and result in a separation of charge.

Therefore, #"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 #"B-F"# bonds have some π character.

Furthermore, the length of the #"B-F"# bonds is shorter than expected for single bonds.

This is consistent with some degree of π bonding in #"BF"_3#.

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