Question

which structures below are plausible resonance contributors for scn⁻? assume the connectivity is as written.

i
ii
iii
iv
incorrect, 3 attempts remaining
a
i and ii
selected
(incorrect)
you have correctly identified that structure ii is a plausible resonance contributor. however, structure i has an excessive separation of charge so it is a very minor resonance contributor.
b
i and iii
c
i and iv
d
ii and iii
e
ii and iv

Explanation:

• Structure I: It has an excessive separation of charge (a +1 charge on S and a -2 charge on N). Such a large separation of charge makes it a very minor resonance contributor.
• Structure II: It has a more reasonable distribution of charge (a -1 charge on N).
• Structure III: The negative charge is on S. But considering electronegativity (N is more electronegative than S in this context for resonance stability), it is not as good as structure II.
• Structure IV: The connectivity is as written (S - C - N). The negative charge on S and the triple bond on CN is a valid resonance form.

Answer:

E. II and IV