The headquarters move is one part of a "two-component move," Sal Ambrosio, president and founding partner of Radnor, PA-based Tactix Real Estate Advisors, tells GlobeSt.com.

The second component calls for "one to two floors, or between 40,000 sf and 60,000 sf in a Downtown building" for an operations center for approximately 185 Sunoco employees, Ambrosio says. His firm negotiated for the locally based petrochemical giant. Discussions regarding the second space are still underway.

Neither Ambrosio nor David Campoli, of REIT Management & Research LLC, HRPT's the local managing agent, would disclose the rental rate for Sunoco's new headquarters to GlobeSt.com. Ambrosio says, "there's been a dramatic increase in vacancy in the Center City trophy market. Companies are fortunate to have a lease expire at the time of a down market. That made this deal at Mellon Bank Center a great value." Campoli adds, "it was a win-win for everybody."

Average asking rental rates for top properties west of Broad St. are $27.83 per sf, according to second-quarter data from the local office of Cushman & Wakefield. No one reached at deadline would play "a guessing game" on how far below that rate the Sunoco lease went.

"The decision to move Sunoco's corporate headquarters to the Mellon Bank Center was made after a thorough review that lasted almost two years," says Sunoco SVP Charles K. Valutas. "We evaluated a number of key factors, including the current market for office space in Center City Philadelphia, and concluded that is it in the best interest of our employees and shareholders to enter this lease agreement," he adds, calling the property "an outstanding building in a highly desirable location."

Locally based Rubenstein Co. owns Ten Penn Center at 1801 Market St. Sunoco has had offices there since 1980 when it was developed by Radnor Corp., a Sunoco subsidiary. In 1991, Sunoco relocated its corporate headquarters there from Radnor Corporate Center in the suburbs.