Dear student
Clo2 does not form dimers due to the following reasons .
odd electron delocalised as it involved in p-pie d-pie bonding ..
in contrast to that as nitrogen doesnot contain d orbital so, the odd electron in N in NO2 is localised , so, due to this reason we can say that no2 dimerise , but clo2 doesnot .
Regards
Arun (askIITians forum expert)