Shaking up its leadership for the third time in six months, Citigroup has named John Havens as the new head of its investment banking and alternative investments group.

The move, a subject of speculation for months, came on Sunday night when Vikram S. Pandit officially selected Mr. Havens, a loyal lieutenant going back to his Morgan Stanley days, to improve the performance of the two units.

Mr. Havens, known as a strong but sometimes abrasive manager, had been in charge of the alternative investments group after Mr. Pandit combined it with the investment bank last fall to form Citigroup’s institutional clients group.

Michael S. Klein, a co-head of Citigroup’s investment bank for the last four years, was given a new assignment with the title of chairman of Citigroup’s institutional client group, but with no day-to-day management responsibilities. Instead, he will move over to Citigroup’s Park Avenue headquarters, where he will continue working directly with clients and lead a committee to help coordinate coverage with the bank’s disparate businesses.

The moves, which emerged from recent business reviews, are the latest in a series of changes that Mr. Pandit has made since taking over for Charles O. Prince III in December. Citigroup has been hard hit by the credit crisis, with more than $20 billion in write-offs and a 60 percent drop in its stock price over the last year.

Besides strengthening its balance sheet, Mr. Pandit has appointed a new chief risk officer and made changes in the way its mortgage operations and global wealth management unit are structured. Even so, investors continue to raise concerns about its financial health as it braces for billions of dollars in additional losses.

The moves also illustrate the organizational and personnel challenges facing Mr. Pandit. “By putting the right talent in the right places, we are enhancing Citi’s ability to meet the needs of our institutional clients,” Mr. Pandit said in a statement.

The appointment was reported Monday by The Wall Street Journal.

The elevation of Mr. Havens is the latest of several senior appointments for Mr. Pandit’s Morgan Stanley colleagues. Don Callahan was named Citigroup’s administrative chief; Brian Leach was recently appointed its chief risk officer.

Mr. Havens has long been at Mr. Pandit’s side. The two were co-founders of Old Lane, which Citigroup bought last year to bring Mr. Pandit aboard, and have worked closely for more than 20 years despite — or perhaps because of — contrasting personalities. Where Mr. Pandit has a more professorial style, Mr. Havens has the command presence of a drill sergeant.

The leadership shake-up moves Edward Kelly into Mr. Havens’s previous role as head of Citigroup’s alternative investment group. He had previously been chief executive of the Mercantile Bancorp, before a brief stint at the Carlyle Group in private equity. His hiring in January led to rumors that Mr. Pandit was thinking of reshuffling the senior management of the Institutional Clients Group.

It also means that James Forese, who had been co-head of the investment bank, will no longer hold that title. He will now report to Mr. Havens as the head of Citigroup’s sales and trading division.

-----------------------------------------------------------------------------------------------