Pena, 25, will report to Washington on Saturday. The Nationals optioned left-hander Matt Chico to Triple-A Columbus following Friday's loss to the Mets.
Trade talks between two clubs starting early this week. It occurred around the same time the Nationals were looking to sign left-hander Jack McGeary, the club's sixth-round pick of the 2007 First-Year Player Draft.
Washington and Boston stopped talking for a few days because the Nats wanted to get the deal done with McGeary first. Once McGeary's deal was completed on Wednesday night, the Red Sox and Nationals resumed talks about Pena. General manager Jim Bowden declined to say who Washington will give up for Pena at a later date. Besides hitting home runs, Pena strikes out often and is considered a mediocre outfielder. Pena will make $1.8 million this season and Boston will be paying all but the pro-rated minimum.
"His power works at any stadium," said Bowden. "He is a good kid with a big heart. He has got to be driven. You have to stay on top of him. You have to get him to work hard.
"I thought we made a good trade. Certainly in Boston's case, Pena is not the guy you carry in a pennant race in the role he had there. He has to play."
Bowden -- who had acquired Pena from the Yankees in 2001 when he was general manager of the Reds -- had pursued him again from the time he became the Nationals' GM in late 2004. This year, according to sources, Bowden was intent on acquiring Pena at a discount because he is having the least productive season of his career. Entering Friday's action, Pena was hitting .218 with five home runs and 17 RBIs on a part-time basis with Boston.
Last season, however, in Pena's American League debut, he hit .301 with 11 home runs and 42 RBIs in 84 games with Boston. Pena was acquired by the Red Sox on March 20, 2006, from the Reds in exchange for right-handed pitcher Bronson Arroyo and cash considerations.
As a member of the Nationals, Pena returns to the National League, where he hit 51 home runs in only 302 games with the Reds from 2002-05, an average of one long ball every 16.2 at-bats. Pena's best season came with Cincinnati in 2004, when he hit .259 with 26 home runs and 66 RBIs despite playing in only 110 games, sharing time in the Reds' outfield with Ken Griffey Jr., Adam Dunn and Austin Kearns.
With increased playing time and at-bats in '04, Pena averaged one home run every 12.9 at-bats, including one every 9.6 at-bats when facing left-handed pitching.
Nationals manager Manny Acta said he plans to platoon Pena in left field with Ryan Church. Acta said Church took the news well, but Church declined to talk to the media about his reduced role.
"This guy has a lot of potential," Acta said of Pena. "He has not been able to play a full season. We know he is going to strike out a lot. Power guys do that. We are very interested in his potential. We need a power bat in here and I think he is going to bring that over here."
It's no secret that the Nationals have been disappointed in Church's offensive production. Entering Friday's action, Church was hitting .265 with 10 home runs and 48 RBIs, while struggling against left-handed pitching. Church has been on the trade block for many months, but Washington hasn't come close to a deal.
"Sometime acquisitions can motivate other players to step up their game. We give players opportunities. No one is ever going to be given a job here," Bowden said. "I think Church has done well lately. ... But this isn't about Ryan. He was not traded. Do I like competition? Sure."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.