The Cavs sent the 30-year-old point guard to Brooklyn to clear cap space for the two-time NBA Champion.
The Cavaliers will clear nearly $22 million in cap space with the trade, allowing them to offer James a max contract to return to Cleveland.
The Nets will acquire young shooting guard Sergey Karasev along with Jack, and will send Marcus Thornton to the Boston Celtics.
The Celtics used their $10.2 million trade exception - which was soon to expire - to acquire Thornton and Cleveland's Tyler Zeller, along with a protected 2016 first-round pick from the Cavs.
With a promising young roster featuring Kyrie Irving and 2014 number-one draft pick Andrew Wiggins, Cleveland will hope that their ability to offer James a max deal will be enough to entice the four-time MVP.
ESPN reports that the Cavaliers are now pursuing Ray Allen and Mike Miller, two of James's favourite players and best friends in the NBA, in the hope of securing his signature.
For the Nets, the trade provides them with a backup point guard to replace Shaun Livingston. Livingston signed a three-year, $16 million contract with the Golden State Warriors after an impressive year in Brooklyn.
The Celtics used their large trade exception, which was set to expire on Saturday July 12, to acquire Thornton's expiring $8.6 million deal. With Rajon Rondo, Brandos Bass, Joel Anthony and possibly Jeff Green - he has a player option next year - all set to become free agents in 2015, Boston could have up to $41.4 million come off the books next summer.