By JULIE SATOW

In New York’s most exclusive office towers, the pendulum of power has swung from landlords, who last year basked in rising rents and competition for prime buildings, to tenants, who are now being offered incentives like prebuilt offices to fill empty spaces. And the leverage held by tenants is likely to continue through this year, market experts say.

“There is no question that during the second half of 2011 leasing velocity slowed in the premier properties,” said Neil Goldmacher, a vice chairman at Newmark Knight Frank. “The fact is, there are a limited number of tenants who will pay three digits in this economic environment.”

At first glance, it appears the trophy office market — which includes 9 West 57th Street, the Seagram’s Building at 375 Park Avenue, 712 Fifth Avenue and a handful of others — ended 2011 on an upswing. According to data from Cushman & Wakefield, there were 46 leases with rents of $100 a square foot or greater in 33 separate buildings last year, compared with just 18 such leases in 13 buildings in 2010. The average starting rent in the trophy market also jumped, reaching $154.75 in 2011 compared with $114.80 in 2010, Cushman & Wakefield reported.

But a closer examination shows that the bulk of those leases, 28, came in the first half of 2011. In terms of the most deals per quarter, the second quarter peaked, with 16 deals with rents of $100 or greater. That fell to just nine leases in the fourth quarter, typically a strong time of year.

And two expensive first-quarter leases at 9 West 57th Street were largely responsible for driving up last year’s overall starting rents: Kohlberg Kravis Roberts & Company’s 64,000-square-foot lease at $147 a square foot and Apollo Commercial Real Estate Finance Inc.’s 32,800-square-foot-lease at $160 and $165 a square foot — an increase caused by a rent bump during the nine-year lease term.

“The message that the $100 deals are back is true, to a point,” said Alan Desino, an executive managing director at Colliers International, who represents tenants. “There are more deals than in 2010 or 2009, but it is still a 40 percent drop from the peak in 2007.”

Several factors caused the slowdown in the second half of 2011, including the shifting employment in the financial services sector, which leases the bulk of the space at the trophy buildings. Wall Street added 8,210 jobs from December 2010 to April 2011. At the same time, pent-up demand stemming from the recession was released, and more tenants entered the market.

“There was real momentum in the market during the first half of the year,” said Tara I. Stacom, a vice chairwoman at Cushman & Wakefield. Landlords snubbed several tenants when larger deals came up; for example, the law firm WilmerHale was bumped from 825 Eighth Avenue in favor of Nomura Holdings America, and Wells Fargo was pushed out of 120 Park Avenue in favor of Bloomberg L.P. The expensive deals signed by K.K.R. and Apollo at 9 West 57th Street also set a high bar for rents.

But from last April to November, the financial services sector shed 5,670 jobs. The sector’s woes were compounded by the earthquake and tsunami in Japan, the stalemate in the federal government over the debt ceiling and the eruption of the European debt crisis. “The result of the negative news meant trading volumes dropped, leading to declining, or at best flat, revenues, and this created job losses on Wall Street,” said Ken McCarthy, a senior economist and senior managing director at Cushman & Wakefield.

The Paramount Group, which owns 712 Fifth Avenue, a modern tower set back behind historic facades on an avenue that is home to hedge funds and private equity firms, began constructing prebuilt offices this summer in vacant spaces on lower floors. That allows tenants to move directly into a space without having to design and build it themselves. The strategy, which can cost landlords anywhere from $90 to $100 a square foot, can be a way to attract tenants without lowering rents.

Now, for the first time, the Paramount Group will pursue this prebuilt strategy in two of the building’s top floors, which have panoramic views of Central Park and the city skyline. “This is the first time we will be doing prebuilts for the tower floors,” said Ted Koltis, a senior vice president at Paramount. “Given everything that went on this summer, in trophy buildings like ours there was a bit of a dropoff in the caliber of the tenant in the market — we saw it quiet down.”

At 9 West 57th Street, owned by Sheldon Solow and perhaps Manhattan’s most exclusive office address, the 49th floor will be carved up into smaller, prebuilt office space. The strategy is twofold, said Scott Panzer, a vice chairman at Jones Lang LaSalle who is in charge of leasing at 9 West 57th Street: “As owners of many buildings, our buying power for constructing units is much greater than it would be for smaller tenants,” he said. “In addition, since we are bulk purchasing these higher-end finishings, we can offer them at a much lower cost.”

Another reason prebuilt spaces make sense now is that many tenants are waiting until the last moment to commit to a lease because of the uncertain economy.

But even with prebuilt offices, some landlords are still having trouble attracting tenants. At 510 Madison Avenue, which is roughly 55 percent vacant, the landlord Boston Properties is marketing prebuilt offices in the middle of the building. While it offers many luxury amenities, 510 Madison’s views of Central Park are limited and its asking rent of $110 a square foot is a 10 to 20 percent premium over comparable buildings, brokers said.

Mr. Goldmacher said it appeared the landlord had been committed to the leasing prices it planned to charge when it acquired the building in 2010, and was using prebuilt offices and other concessions to avoid lowering rents. Boston Properties declined to comment, citing a quiet period for the company, which is about to announce earnings.

Other shifts in the market include an increasing number of short-term renewals. “Right out of the gate tenants are looking for two- to three-year deals rather than five- or 10-year leases,” said Cynthia Wasserberger, a managing director at Jones Lang LaSalle.

“The high-end market is very slow,” said Ben Friedland, a senior vice president at CBRE. “We are finding that unless they really need to make a move, tenants won’t and defer it by doing short-term renewals.”

Tenants are also beginning to discuss sublease space, which is typically less expensive than office space leased directly from a landlord. Ms. Stacom said she expected the market to continue to improve for landlords, albeit slowly. “The foot came off of the accelerator,” she said, “but the foot is not on the brakes.”