Why nucleophilic attack is not possible on n+ atom.... is that due to steric hindrence

The positive charge on N is the donation of it's lone pair from a complete octet. a nucleophile is looking for an acidic centre while nitrogen is an electron rich base here fulfill...

Thank you brother

yes, nucleophile has a lone pair of electrodes which is repailed by negative charge of O atom. but in case of 1,3 dipolar species, there is more distance between + and - charge so ...