As others have said, Edmundson is likely not available for anyone the Penguins would be willing to part with. He is a borderline top pairing defenseman, somewhere between a high end #3 and low end #2 right now...and he's only 25 and still a RFA. Plays big 5v5 numbers against top lines, one of our best PK guys, very solid in his own zone, very good skater for his size, decent moving the puck, strong shot and sneaky good offensive instincts, etc etc etc.
He is still underrated IMO, but for a while there Edmundson was criminally underrated (even by Blues fans).
Blues could retain a bit on either Bouwmeester of Gunnarsson if they interest the pens at all. Some Blues fans will disagree, but I think Bouwmeester can still be a legit top 4 guy if he can stay healthy next year. Prospects like Walman or Schmaltz could be available as well. FWIW, I've thought Schmaltz would be an excellent fit in Pittsburgh for a while now.
Love Edmundson and think he's pretty underrated... but I think we'd have to find a way to expand this and get Sobotka... otherwise we're back looking for a quality center.
Sobotka + Schmaltz for Brassard + conditional pick (condition = re-signing in St. Louis)?