Sources told ESPN that a medical issue is holding up the deal from being finalized.
Bruce, 29, has spent his entire nine-year career with the Reds. He leads the National League in RBIs this season with 80 to go along with 25 homers and a .265 batting average. He becomes the first player traded during the season while leading the league in RBIs since the stat became official in 1920, according to Elias Sports Bureau.
Bruce is a lifetime .249 hitter with 233 home runs and 718 RBIs.
In the trade, which is pending physicals, the Reds get a number of prospects, including 23-year-old outfielder Brandon Nimmo, according to ESPN's Adam Rubin. Nimmo has played 18 games with the Mets this season. The 2011 first-round draft pick batted .336 in Triple-A this season.
Bruce's contract has a team option next season for $13 million, with a $1 million buyout.
Why did the Mets need Jay Bruce? For starters, they rank last in the majors in batting average with runners in scoring position at .206.
As each deal goes down, we'll give you our report card — then ask you to vote on which team hit the jackpot and which team got the short (or at least shorter) end of the stick.
The Mets were not on Bruce's no-trade list, which included the Red Sox, Rays, Yankees, Blue Jays, Diamondbacks, Marlins, A's and Twins.
The Mets boast one of the best pitching staffs in baseball, led by Noah Syndergaard and Jacob deGrom, but they are only 13th in the NL in runs scored. Last season, they were in a similar position and acquired Cespedes, who sparked them to the World Series.