How do resonance structures affect bond length?

1 Answer
Aug 5, 2014

The bond lengths in the resonance hybrid should be an average of the bond lengths in the separate contributors.

Consider for example the nitrite ion, NO₂⁻. It has two resonance contributors.

The resonance hybrid has two identical N-O bonds. They share a double bond and a single bond between them, so the average bond order is 1.5.

We would predict that the N-O bond length should be halfway between a single bond and a double bond.

The N-O single bond length is 136 pm. The N=O double bond length is 115 pm.

The average of these bond lengths is 126 pm.

The observed bond length in NO₂⁻ is 125 pm.