The first building of the two-building, 500,000-square-foot Rockledge Center office complex, located near the intersection of the Beltway and I-270 in Bethesda, has been completed, and the second is well under way, according to the architectural firm of Clark Tribble Harris & Li.

IBM Corp. will occupy all of One Rockledge Center. Two Rockledge Center, which is scheduled for completion in January, will be leased to other corporate tenants.

The twin, 10-story buildings stand on an elevated 12-acre site in Rockspring Park, near the intersection of the two highways.

The $75-million complex was designed by Clark Tribble Harris & Li and is being developed jointly by Spaulding & Sly and IBM Corp.

Other recent developments in commercial realty in the Washington metropolitan area:

* Centennial Development Corp. reports the signing of five leases totaling 17,000 square feet at Centennial Plaza I, an 18,300-square-foot office building located on Old Courthouse Road in Tysons Corner: Mohawk Data Sciences Corp., a computer company, for 5,000 square feet; Mandex Inc., a minority-owned technology services firm, for 4,400 square feet; Eaton Design Group Inc., an interior design firm, for 3,000 square feet; Comtravest International Inc., a trading services firm, for 2,900 square feet and Smith Segreti Tepper McMahon Harned, an architectural firm, for 1,600 square feet.

* Construction is under way on a 90,000-square-foot, three-story building at the Johns Hopkins University Applied Physics Laboratory near Columbia, Md. James Goldstein and Partners of Millburn, N.J., are the architects, and Nooco Contractors of York, Pa., are the general contractors.

* Federal Data Corp., a computer systems integration firm, will combine its offices in 65,000 square feet of office space it has leased at One Bethesda Center, located at 4800 Hampden La., just of Wisconsin Avenue in Bethesda. CAPTION: Picture, Model of the $75 million Rockledge Center being constructed on 12-acre site.