Remove the contracts cause they are quite similar. (I understand one is a year less which does have some impact)
Lets compare the players:
Emelin (31 year old defenseman)
2015-16 : 72 games - 0 goals - 12 assists
Based on TOI, I would say 2nd pair defenseman with limited upside due to age
Hartnell (34 year old winger)
2015-16 : 79 games - 23 goals - 26 assists
Produces like a 2nd line winger (borderline 1st line) also with limited upside, but goal numbers for last 4 full seasons (37, 20, 28, 23) show he is still a high producer.
If Hartnell even produces for one more season this is a huge win for Montreal as his trade value skyrockets as the number of years remaining on contract going down. Emelin has little to no value to Columbus as he is too old for the core and would only keep younger players out of the lineup without adding much in terms of quality play.
Strong decline for CBJ - offer something other than your throw-aways for Hartnell and you could get some traction.