how do no2 ,cn ,give -i effect

In no2

1oxygen is connected to nitrogen by double bond and one is attached by single bond and this oxygen carries negative charge.....and nitrogen itself carries positive charge...therefore when the nitrogen oxygen double bond breaks heterolytically nitrogen gets two + charge hence it is an urgent need of electrons therefore -i effect.....

similarly carbon nitrogen bond in CN breaks heatrolytically and carbon gets two+ charge now same thing happen again..