Assign formal charges to each atom in the resonance forms of the cyanate ion (OCN^-). Which resonance form is likely to contribute most to the correct structure of OCN^-?

Question

Assign formal charges to each atom in the resonance forms of the cyanate ion (OCN[latex]^{-}[/latex]). Which resonance form is likely to contribute most to the correct structure of OCN[latex]^{-}[/latex]?

[latex]\overset{A}{[:\overset{..}{\underset{..}{O}} —C ≡N:]^{-}}[/latex] [latex]\overset{B}{[:\ddot{O} =C —\ddot{N}:]^{-}}[/latex] [latex]\overset{C}{[:O≡C —\overset{.. }{\underset{..}{N}} :]^{-}}[/latex]

Accepted Answer

Calculate the formal charge on each atom by finding the number of valence electrons and subtracting the number of lone pair electrons and one-half the number of bonding electrons.

	A [latex][:\overset{..}{\underset{..}{O}} —C≡N:]^{-}[/latex]			B [latex][:\ddot{O} =C—\ddot{N}:]^{-}[/latex]			C [latex][:O≡C—\overset{.. }{\underset{..}{N}} :]^{-}[/latex]
Number of valence e[latex]^{-}[/latex]	6	4	5	6	4	5	6	4	5
-number of lone pair e[latex]^{-}[/latex]	-6	-0	-2	-4	-0	-4	-2	-0	-6
-[latex]\frac{1}{2}[/latex](number of bonding e[latex]^{-}[/latex] )	-1	-4	-3	-2	-4	-2	-3	-4	-1
Formal charge	-1	0	0	0	0	-1	+1	0	-2

The sum of all formal charges for each structure is -1, as it should be for a 1- ion. Structures A and B have the least amount of formal charge and are therefore preferable over structure C. Structure A is preferable to B because it has the negative formal charge on the more electronegative atom. We thus expect structure A to make the biggest contribution to the resonance forms of the cyanate ion.

Question 9.8: Assign formal charges to each atom in the resonance forms of...

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