(Updates stock price at opening bell.)After weeks of back and forth, Exelon ( EXC) has sweetened its offer for NRG Energy ( NRG) in its ongoing hostile bid to take over the company. Exelon increased its bid on Thursday, citing $1.5 billion in new synergies. The company said it would now offer 0.545 of its own shares for each NRG share in the all-stock transaction. The deal is now valued at more than $7.4 billion. Exelon initially offered an exchange ratio of 0.485 last fall. Exelon shares were trading down 1.7% to $50.70 immediately after the opening bell Thursday. In recent weeks, New Jersey-based NRG has said Exelon's overtures undervalued the company, leading to court filings and comments in the media. Exelon, the biggest operator of nuclear power facilities in the country, also is pushing its own slate of directors at NRG's annual meeting in July in its attempt to make the deal's case directly to NRG shareholders. "We listened to NRG investors and balanced their views with the best interests of Exelon shareholders," Exelon CEO John Rowe said in a press release Thursday. "This is our best and final offer, and we will use the time leading up to the NRG annual meeting on July 21 to communicate the value of our new offer to NRG shareholders, encouraging them to vote for nine new independent directors who can unlock that value."