PITTSBURGH — PNC Financial Services Group Inc., one of the nation's largest regional banks, said Monday it is buying Baltimore-based Mercantile Bankshares Corp. for about $6 billion, significantly expanding its presence in the Mid-Atlantic region.

The Pittsburgh company said the cash-and-stock deal will further its growth along the affluent New Jersey-to-Washington corridor, adding branches in Maryland, the District of Columbia, Virginia, Pennsylvania and Delaware.

"I think it's fairly self-evident that the combination of Mercantile and PNC will be a mid-Atlantic powerhouse," said James Rohr, PNC's chairman and chief executive.

The regional footprints of the banks fit well together, he told analysts during a conference call. "With their 240 branches, PNC will have market coverage of the wealthy East Coast corridor from the Hudson River to the Potomac," Rohr said.

The transaction will make PNC the second-largest bank by deposit market share in Maryland and Delaware and is expected to accelerate growth in Washington and Delaware.

Mercantile is expected to start adding to PNC's earnings per share in 2008 with an estimated 15 percent internal rate of return.

Last year, PNC got a foothold in the Washington market with its $643 million acquisition of Riggs Bank, which was headquartered in the nation's capital.

With the Mercantile acquisition, 69 percent of PNC's branches will be east of the Appalachian Mountains, Rohr said.

It will give the bank a presence in Howard County, Md., among the country's 10 most affluent areas by median household income, and other parts of Maryland, he said.

Based on PNC's closing stock price of $73.60 on Friday, the deal values shares of Mercantile at $47.24 apiece, a 28.4 percent premium over their closing price of $36.78 on Friday.

Mercantile shares rose $8.04, or 22 percent, to $44.82 in afternoon trading on the Nasdaq Stock Market. PNC shares fell $3.46, or 4.7 percent, to $70.14 in afternoon trading on the New York Stock Exchange.

PNC expects the deal to close in the first quarter of 2007.