PPT Slide

16

(1) small FC (whether + or -) are preferable to large ones.

(2) like FC charges on adjacent atoms is not desirable.

(3) more negative FC should reside on atom with more EN.

Consider the cyanate ion, NCO – (16e–):

Three points govern selection of the most important resonance structures based on formal charge (FC):

Eliminate A because it has larger formal charges than the others.

Forms B and C have the same magnitude of formal charges.

Form C has a –1 FC on O, which is more electronegative than N.

Contributions to the resonance hybrid are: C > B > A

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Notes:

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