When drawing CN, how do I know it has a triple bond instead of double bond?

[saoj]

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On a TBR passage I was asked to calculate the formal charge of Cr(CN)6. That requires me to first know the formal charge of CN. The solution states it is -1 like it was as obvious as 2 + 2 = 4. 😕

So my confusion:

1) To calculate the formal charge of CN I have to draw the Lewis structure with lone pairs and bonds, correct?

2) How do I know that CN has a triple-bond instead of a double-bond?

I guess if I cannot correctly draw CN, I will not be able to calculate its correct formal charge, right?

What is the best way of attacking this problem?

 

[MedPR]

On a TBR passage I was asked to calculate the formal charge of Cr(CN)6. That requires me to first know the formal charge of CN. The solution states it is -1 like it was as obvious as 2 + 2 = 4. 😕

So my confusion:

1) To calculate the formal charge of CN I have to draw the Lewis structure with lone pairs and bonds, correct?

2) How do I know that CN has a triple-bond instead of a double-bond?

I guess if I cannot correctly draw CN, I will not be able to calculate its correct formal charge, right?

What is the best way of attacking this problem?

You should know CN has a triple bond the same way you know that CH4 is all single bonds. CN- is a nitrile group, also something you should know.

If, however, you don't know, you can figure it out.

C wants to make 4 bonds, Nitrogen wants to make 3 bonds. Obviously you can't have a quadruple bond, but you can make a triple bond so at least nitrogen is happy.

 

[saoj]

C wants to make 4 bonds, Nitrogen wants to make 3 bonds. Obviously you can't have a quadruple bond, but you can make a triple bond so at least nitrogen is happy.

Cool. With 3 bonds at least N is happy. With 2 bonds both are unhappy.