What is the molecular geometry of CN-?

Answer 1

According to the assignment of formal charges, N should be 0, C is 0 and O is -1. This is the most stable bonding between the atoms as O is the most electronegative hence it should have the -1 formal charge. From this, we know that there is a triple bond between N and C; a single bond between C and O. Hence there is no lone pair around the C atom. From VSEPR, we know that this molecule would then be linear.

Answer 2

There are two possible structures for NCO-

:N---C-O::: or ::N=C=O:: (--- is a triple bond)

The more "correct" structure I want to say is :N---C-O::: because when you assign formal charges, the more electronegative atom, O, will have the formal charge of -1.