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I was having trouble drawing the Lewis structure for $\ce{NCO^-}$. I found this image online:

enter image description here

But why is structure number one preferred over structure number two? Why does carbon prefer to have a triple bond with nitrogen, rather than have a double bond with nitrogen and a double bond with oxygen?

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If you count electrons and determine the formal charge on each atom, you find that in structure #1, the negative charge is on the oxygen. Do the same exercise for structure #2 and you find that the negative charge is on nitrogen. Since oxygen is more electronegative then nitrogen, the negative charge is more stable when its on the oxygen atom. Therefore, structure #1 is more stable (preferred) than structure #2.

Structures #3-#5 are even less stable. They either violate the octet rule (#4, #5) and / or have charges (sometimes multiple) on more than one atom (#3-#5).

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    $\begingroup$ When talking about resonance structures, I think stable is the wrong word. I agree with your analysis in terms of contribution. Therefore #1 should have the highest contribution of all. $\endgroup$ – Martin - マーチン Jul 16 '15 at 18:58
  • $\begingroup$ @Martin Perhaps this is just semantics, but I disagree with your comment. Why is one resonance structure a larger/smaller contributor than another - because it is lower/higher energy than another - e.g more/less stable than another. "It" referring to the resonance structure as if it were an isolated, existing structure, which in reality it is not. $\endgroup$ – ron Jul 16 '15 at 19:04
  • $\begingroup$ I agree it is a question of semantics. Since individual resonance structures are not real, none of them are stable, only the complete set of structures is. However, the electronic ground state can be described as a linear combination of configurations, which can be chosen to represent individual Lewis structures. From the coefficients of these individual configurations we can see a contribution to the ground state. I just can't bring myself to talk about a stable resonance structure, since I know they are individually not stable in the sense of the definition of the word. $\endgroup$ – Martin - マーチン Jul 17 '15 at 9:02

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