Deutsche Bank AG and Credit Suisse Group AG led bond dealers that entered contracts to buy and sell government-backed mortgage securities to the Federal Reserve as the central bank acquired $1.25 trillion of the debt beginning in January 2009. The purchases, aimed partly at bolstering the housing market by lowering financing costs, represented the largest portion of an unprecedented $1.72 trillion in debt-buying by Chairman Ben S. Bernanke’s Fed during its first round of quantitative easing. The central bank announced a second round, dubbed QE2, last month.