The Islanders got Leddy from the Chicago Blackhawks in exchange for ex-Toronto Marlies defenceman T.J. Brennan, defensive prospect Ville Pokka and the rights to goaltender Anders Nilsson, who's a restricted free agent.
New York got Boychuk from the Boston Bruins for a second-round pick in each of the next two drafts and a conditional third-rounder that only goes if the Islanders trade him to an Eastern Conference team this season.
Leddy, 23, was sent out of Chicago so that the Blackhawks could free up some salary-cap space. Chicago signed winger Dan Carcillo to a league-minimum deal early Saturday.
Boychuk, 30, was similarly a victim of the cap in Boston. He's set to be an unrestricted free agent next summer.