I have been trying to link a few raised questions as duplicates, as I have seen answers to them on the main stackoverflow. However when I suggest closure and try to link to stackoverflow I am unable to do so?
Without knowing what questions you are referring to, first consider if we should be sending valid questions/users off to another site.
But on a technical note, it is not possible to close a question as a duplicate of one on another site. Migration paths are not added until a site becomes a lot more established, so if a question truly belongs somewhere else, you can 'flag' it for Moderator attention. But even then, migrations are fraught with problems so we don't generally recommend them anymore.
Migrations are used sometimes (should be rarely) to preserve long-standing content. But if the author simply asked a question on the wrong site, you should politely ask them to re-post their questions in the context of the correct site.
But posts sent off to another site means removing everyone's reputation that goes with it, and orphaned posts are often greyed out as anonymous when the authors don't have an account on the target site. You also end up with odd-looking comments that don't quite fit, and the posts/voting may not fit tone and posting convention of the target site. So generally speaking, migrations do not work that well, so they are generally not recommended as a routine way to shuffle questions off to another site.