Citigroup Inc. said on Monday it agreed to buy Automated Trading Desk for $680 million to increase its ability to allow clients worldwide to trade stocks electronically. The largest U.S. bank said it will pay $102.6 million in cash and issue about 11.17 million shares. It expects the transaction to close in the third quarter, pending regulatory approval.

Automated Trading Desk has about 120 broker-dealer customers and trades on average more than 200 million shares daily, comprising 6 percent of volume on both the New York Stock Exchange and Nasdaq. Citigroup said it intends to expand Automated Trading Desk's platform worldwide, as more customers demand the ability to trade quickly and at low cost. The New York-based bank operates in more than 100 countries.

Steven Swanson will remain chief executive of Automated Trading Desk. The firm will keep its Mount Pleasant, South Carolina, headquarters and operate as a unit of Citigroup's global equities business.

The transaction is the latest in a series of acquisitions for Citigroup since it was freed 15 months ago from a Federal Reserve order that restricted mergers until the bank improved its internal controls. Chief Executive Charles Prince is using acquisitions to supplement internal growth, and boost revenue faster than costs on a consistent basis amid heavy shareholder pressure. Citigroup's own bankers and the law firm Skadden, Arps, Slate, Meagher & Flom LLP advised Citigroup. Financial Technology Partners LP and the law firm Blank Rome LLP advised Automated Trading Desk.