An ion of nitronium, NO+2, is a cation. Because of its tetravalent nitrogen atom and +1 charge, it is an onium ion, similar to ammonium in that respect. It is produced by removing an electron from a molecule of paramagnetic nitrogen dioxide or by protonation of nitric acid (with removal of H2O).
In NO2+, by the double bond, the nitrogen atom is bound to one oxygen while the coordinate covalent bond is bonded to the other oxygen atom. There is thus, an unpaid electron that gets ionized by the positive charge on the nitrogen atom. There is not an octet around the nitrogen in NO2+, but it is an electrophile.