The rejection forestalls, at least for now, the creation of what would be the largest power company in the nation.

The Exelon Corporation says its unsolicited bid to buy NRG Energy would create the largest power company in the country, in terms of assets, market capitalization and generation capacity.

The Exelon Corporation said late Sunday that it had offered to buy NRG Energy for $6.2 billion.

Exelon, the electric company based in Chicago, will promise to cut greenhouse gas emissions by 2020 by an amount larger than its total emissions in 2008.