CNO- exhibits resonance. AsI3 do not exhibit resonance. The electrons in the double bond (C=O) can move to the other C-O bond.

What is the best resonance structure for CNO?

Carbon has the negative charge (-3) while oxygen (which is more electronegative) has a (+1) formal charge. Therefore, the best Lewis structure for fulminate ion is structure B.

What type of bond is CNO?

The central atom for CNO should be C, since it has the lowest ionization energy. The Lewis Structure should then have one covalent bond between C and O, and 3 covalent bonds between C and N. Also, O has 6 lone pair electrons and N has 1.