Kirkland & Ellis is representing Constellation Energy Corp. in its pending acquisition of Calpine Corp. for $16.4 billion in cash and stock, a deal that will create the nation's "largest clean energy provider," while Calpine turned to Latham & Watkins and White & Case.

Constellation, based in Baltimore, and Houston's Calpine announced a definitive agreement on Friday, which is expected to close within a year.