NEW YORK — John J. Mack, president and chief operating officer of Morgan Stanley Dean Witter & Co., announced on Wednesday that he will leave the firm on March 21.

Morgan Stanley’s board of directors accepted his resignation and named Robert G. Scott, the firm’s chief financial officer, to succeed him, the investment banking company said. Mr. Scott will run the investment bank alongside Philip J. Purcell, the company’s chairman and chief executive officer.

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