Why is NCO- more stable than CNO-?

Question

 I know that NCO- is more stable I think it has to do with the formal charge but I just don't understand why. Please help!!

Answer 1

You're right that it has to do with formal charge. The less formal charges a molecule has, the more stable it is. The key to understanding this problem is understanding of Lewis structures.

 

I can't really show it here, but when you correctly draw out the Lewis structures for NCO^-, you'll see that in one form, the formal charge of oxygen is -1, nitrogen is 0, carbon is 0. The other resonant structure has the negative charge on nitrogen while carbon and oxygen are 0. This means in NCO^- there is only one negative charge flipping back and forth between nitrogen and oxygen. The negative charged is smeared throughout the molecule which is stabilizing.

 

In the lewis structures of CNO^-, the three forms have a bunch of formal charges. One form has carbon -2, nitrogen +1, oxygen 0. The second form has carbon -1, nitrogen +1, oxygen -1. And the third form has carbon -3, nitrogen +1, oxygen +1. So no matter what resonance tries to do in CNO^-, there are destabilizing formal charges everywhere.

 

I hope that's clear enough. :)