Citigroup Inc. said Steven Black, global head of equities at its Salomon Smith Barney Inc. securities unit, resigned in an ongoing reshuffle at the top of the world's biggest financial services company. Separately, Citigroup, whose stock has been hampered recently by concerns about the integration of Citicorp and Travelers Group into Citigroup, said it was buying back $2 billion of its own stock. Black, 46, who joined Smith Barney in 1974, played a pivotal role in building the firm's equities business. His departure could foreshadow other resignations from leading Citigroup executives as the company struggles to put its different banking operations together, analysts said. The changes in the upper ranks at Citigroup have heightened Wall Street's concern about the integration of Salomon Smith Barney with Citicorp's corporate bank, forcing the company to publicly defend its merger. Citigroup shares fell $1 to close at $45 on the NYSE.