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It has long been the $45 billion corporate buyout that should never have happened.

But it did. And now a deal that defined the money and power of the golden age of private equity has gone bad the way things often do: slowly, and then all at once.

The TXU Corporation, the Texas energy giant that was taken over in a record-shattering buyout in 2007, finally collapsed into a long-awaited bankruptcy early Tuesday.

On the surface, the long, slow decline of the company, renamed Energy Future Holdings, has caused few ripples, though it is the state’s largest electricity generator and provides power to three million customers. The private-equity owners wrote their financial stakes down to almost zero years ago, and many debt investors sold their holdings at a loss. And thanks to behind-the-scenes negotiations in recent weeks, a group of hedge funds and other investors led by the financiers Leon D. Black of Apollo Global Management and Bruce A. Karsh of Oaktree Capital, who bought pieces of the company’s debt on the cheap, is walking away with a big prize: the power generation business.

But the bankruptcy of Energy Future Holdings represents a reckoning of sorts for some of the biggest names in private equity, a signature Wall Street business that over the last decade drastically reshaped corporate America and in the process took what it meant to be “rich” to a new level.

The bankruptcy is the 11th largest in history and one of the largest failures of a company in private-equity hands.

Several deals involving United States companies that were taken private during the buyout boom era, from 2005 to 2007, have turned out to be big winners. The buyouts of the hospital giant HCA, Hilton Hotels and the energy company Kinder Morgan yielded big returns for the buyout kings and their investors.

Still, other prominent deals of the era, including the $27 billion buyout of the First Data Corporation in 2007, the $23 billion acquisition of Clear Channel Communications in 2008 and the $17 billion purchase of Freescale Semiconductor in 2006, are still struggling to manage the mountains of debt that the deal makers piled on them in industries that have not yet rebounded.

Analysts say such broad disparities — big gains but potentially huge losses — do not represent the type of steady returns private equity sells to its pension-fund investors. Moreover, they say, the era’s biggest deals could reflect a hubris that led to an increased appetite for risk.

“The dangers of these boom-era deals is that they were taking on more risk on the financial side, in terms of increased leverage, and taking on more risk on operations based on their models,” said Erik M. Gordon, a professor of law and business at the Ross School of Business at the University of Michigan. “If you drive your car at 50 miles an hour and hit a pebble, you might feel the bump. If you’re driving the car at 150 miles an hour and hit the pebble, you’ll end up in a tree.”

But the outcome of such gambles was not nearly as severe as many predicted just a few years ago, for which some analysts say those who placed the bets owe a big debt of gratitude to the Federal Reserve.

Because of the extraordinary measures the Fed has taken in recent years to hold down interest rates, investors have moved into riskier bonds to earn higher yields, allowing the companies owned by private-equity firms to refinance their debt.

“The Fed created a very friendly environment for borrowers that allowed them to roll over their debt and delay their potential crisis points, whether or not their operations were improving,” said Martin Fridson, the chief investment officer of the money management firm Lehmann, Livian, Fridson Advisors.

This rollover of debt, often derisively nicknamed “extend and pretend” or “delay and pray,” has worked. A gigantic wall of debt — hundreds of billions of dollars — that was once due in 2014 has been pushed back to peak in 2018, according to some estimates.

That delay has given some companies that suffocated during the recession under a mountain of debt the time to recover along with the rebound in the United States economy.

Still, not all of the megadeals from private equity’s golden age will end well. As Energy Future illustrates, even the most dexterous financial maneuvers aren’t enough if the underlying business is floundering.

In 2007, after months of courting environmental groups, lobbying the Texas legislature and waving hundreds of millions of dollars in fees in front of salivating Wall Street banks, a group of high-powered private-equity barons, including Kohlberg Kravis Roberts, the Texas Pacific Group and the private-equity arm of Goldman Sachs, bought TXU in a deal valued at the time at more than $45 billion.

Investors including Warren E. Buffett gobbled up pieces of the nearly $40 billion raised to complete the takeover.

But the buyout, which was in effect a giant bet that natural gas prices would continue to climb, soon faced trouble as natural gas prices plummeted.

By 2009, K.K.R. started to write down the value of its stake in Energy Future. By early 2012, it valued its stake at a mere 10 cents on the dollar.

Finally, after months of on-again-off-again talks among the power company, its owners and a dizzying hierarchy of creditors, Energy Future went into Chapter 11 protection on Tuesday with a plan intended to stave off months of potentially rancorous fighting in court over pieces of the company.

Under the restructuring, the equity stakes held by K.K.R. and its buyout partners are likely to be wiped out completely.

Instead, Mr. Black of Apollo and Mr. Karsh of Oaktree walk away with the winning hand. After buying bonds owed by one division of Energy Future, they and others will effectively swap $23 billion worth of debt for control of the unregulated power company Luminant Generation and the retail provider TXU Energy, according to the terms of the proposed restructuring agreement. The investment firms caused an uproar during parts of the negotiations when they considered taking over Luminant and TXU Energy in a maneuver that could have saddled Energy Future with a tax bill of at least $7 billion, an unaffordable amount. That would potentially have drawn the ire of the Internal Revenue Service, according to people briefed on the matter.

As it turned out, the shining jewel of the buyout boom was done in by the mundane.

“The company was on the wrong side of the energy markets, and they had a capital structure that just wasn’t sustainable in light of how the business has gone,” Mr. Fridson said.

Energy Future Holdings bankruptcy petition