# Cyanide's Lewis structure - electronegativity or octet rule?

The formula for the cyanide ion is $\ce{CN-}$. That gives us a total of ten valence electrons to work with. There are two obvious ways to build the Lewis structure.

$\ce{:^-C#N:}$

$\ce{:C=N^-::}$

(I hope that was clear. I'm not sure how to make ASCII Lewis structures. In that second example, the nitrogen has two lone pairs.)

In the first example, each atom has a complete octet, but the carbon has a formal charge of -1. In the second example, the carbon atom does not have a complete octet, and the nitrogen (the more electronegative atom) has a formal charge of -1.

My understanding is that when formal charge cannot be avoided, negative formal charge should reside on the most electronegative atom.

Which rule takes precedence over the other? Using cyanide as an example, should I:

1. Follow the octet rule and go with the first example?
2. Follow the electronegativity rule and go with the second example?
3. Treat them as resonance structures?
• Octet rule should always take priority. I do not know of any counterexamples to this. (In fact, they are resonance structures. The same rules - octet, electronegativity - allow you to judge which is the greater resonance contributor. In this case the first resonance form contributes more.) Oct 18, 2016 at 23:27
• Related and related.
– Jan
Oct 19, 2016 at 12:45

Which rule takes precedence over the other? Using cyanide as an example, should I:

1. Follow the octet rule and go with the first example?
2. Follow the electronegativity rule and go with the second example?
3. Treat them as resonance structures?

You should treat them as resonance structures (both are valid Lewis structures, they have the same connectivity of atoms, and they are related by one of the usual electron-pushing operation, i.e. double bond turning into single bond and lone pair).

Then, you can look at the other rules to make a guess which resonance structure should have higher weight. The octet rule takes preference over the formal charge rule:

The structure on the left follows the octet rule for both atoms, whereas the one on the right has only six electrons associated with the carbon atoms. So if you only want to write a single Lewis structure, the one on the left is better. If you have space, write the pair of resonance structures with a double arrow ($$\ce{<->}$$) between them.

Combining the cyanide ion with a hydrogen ion, there are two resulting molecules that exist, hydrogen cyanide (HCN) and hydrogen isocyanide (HNC). Hydrogen cyanide, for which you can write a Lewis structure that follows the octet rule, is more stable than hydrogen isocyanide (for which you can't). Once these two molecules get deprotonated, they yield the same ion, cyanide, described by the two resonance structures.