* TSX down 24.86 points, or 0.2 percent, at 12,207.07 * Investors await Fed decision, possible commodity boost * Energy companies gain as Mideast tensions boost crude TORONTO, Sept 13 (Reuters) - Canada's main stock index slipped on Thursday as investors waited to see whether a U.S. Federal Reserve policy meeting would conclude later in the day with measures to support the economy of Canada's largest trading partner. Any substantive U.S. economic stimulus efforts would likely boost the prospects of a collection of commodities, which in turn, would benefit the resource-rich Toronto index. "Markets are on hold right now, until the Fed announcement and I think that will set the tone for the rest of the day and into perhaps the rest of the month," said Philip Petursson, director of the portfolio advisory group at Manulife Asset Management. The Fed appears set to launch a third round of unconventional monetary stimulus while signaling that a weak U.S. economy may warrant ultra-low interest rates for at least another three years. "You are likely to see the market move on it and it will have a more positive impact to the commodities market," Petursson added. Energy companies benefited as the price of oil rose on a combination of escalating Middle East tensions and expectations the Fed would move on monetary easing. But most other sectors slipped, and by 10:30 a.m. ET (1430 GMT), the Toronto Stock Exchange's S&P/TSX composite index was down 24.86 points, or 0.2 percent, at 12,207.07. Among individual companies, Transat AT Inc shares jumped almost 10 percent to C$4.50 after the travel operator swung to a profit as cost-cutting bore fruit, and said it expects margins to continue to improve. Shares of medical isotope provider Nordion Inc pared some recent losses to gain 1.8 percent to $C6.99 on news it will reorganize its business into two distinct units and appoint a private equity executive to its board. The stock had slumped last week as Nordion suspended its dividend after an arbitration panel rejected its claim for damages from its main supplier. Macdonald, Dettwiler and Associates shares fell 7.2 percent after the surveillance and satellite company said the U.S. Department of Justice was seeking more information about its planned acquisition of Space Systems/Loral Inc.