(Correction: The original article misstated when Charles Prince, the former chief executive of Citigroup, declined an invitation to the Festival of Thinkers in Abu Dhabi. It was in September, not "just days" before the festival began on October 21.) 

 
An investment by Abu Dhabi is being hailed today as a huge boost for Citigroup, but it might nearly have been a missed opportunity.

Charles Prince, then the chairman and chief executive officer of Citigroup, was invited by Abu Dhabi to participate in October's biennial Festival of Thinkers, which brought 18 Nobel laureates and some 175 top business leaders, scholars, media executives, and artists to the oil-rich Persian Gulf state for four days of celebrating and conviviality.

The participants had access to top decisionmakers in Abu Dhabi, whose proven oil reserves are estimated to last for another 300 years.

The invitation to Prince came from Sheik Nahayan Mabarak al-Nahayan, the festival's founder and the minister of higher education and scientific research of the United Arab Emirates.

The 52-year-old Sheik Nahayan also happens to be the chairman of the Abu Dhabi Group, a private investment fund that is one of the largest investors based in the United Arab Emirates.

The market value of Sheik Nahayan's investments—in banks,   telecommunications, real estate, media (he owns a majority stake in CNBC Pakistan, for example), and financial services, among other sectors—is well above $10 billion, and the Oxford-educated sheik is reportedly on the lookout for more opportunities, particularly in the United States.

Prince, however, declined the invitation. After some inquiries, a spokesman replied on September 20 that the chief executive had prior commitments.

The C.E.O., to be sure, was struggling with a bank in crisis. Citigroup took a write-down of $5.9 billion  on investments tied to subprime mortgages in October, and was soon estimating it would need to take billions more. Prince resigned on November 4 after having lost the support of backers like Prince Walid bin Talal of Saudi Arabia.

Still, had the embattled Citigroup C.E.O. been able to travel to Abu Dhabi for the festival, he almost certainly could have obtained assurances of badly needed funds—a lifeline that might have saved his job as well.

Abu Dhabi officials have long been considering such investments as rising prices for crude oil yield unprecedented wealth for their state and other OPEC members. Indeed, the six countries that make up the Gulf Cooperation Council expect their combined budget surplus in 2007 to exceed $700 billion.

And although Sheik Nahayan's invitation to Chuck Prince had not hinted at an investment by Abu Dhabi, the complex web of decisionmaking in the United Arab Emirates is such that Prince most certainly would have found interested investors had he attended the Festival of Thinkers.

This became clear late Monday night, when Citigroup announced that the investment arm of the Abu Dhabi government had agreed to pump $7.5 billion into the bank. This will give the Abu Dhabi Investment Authority—the government's investment arm—a 4.9 percent equity stake, slightly more than that held by Prince Walid.

Abu Dhabi's cash infusion at once strengthens Citigroup's capital base, which has thinned on account of costly acquisitions, the collapse of the subprime mortgage market, and the resulting turmoil in the credit markets.

"This investment reflects our confidence in Citi's potential to build shareholder value," said the Abu Dhabi Investment Authority's managing director, Sheik Ahmed bin Zayed al-Nahyan.

Founded in 1976 by the late president of the United Arab Emirates, Sheik Zayed al-Nahyan, the Abu Dhabi Investment Authority is one of the largest institutional investors in the world. It has some $900 billion in investments and another $1 trillion available in cash from oil income.

Abu Dhabi is the wealthiest of the seven principalities that make up the federation of the United Arab Emirates because it possesses virtually all of the country's crude oil and natural gas. The U.A.E. pumps 2.2 million barrels of crude oil a day.

The Abu Dhabi Investment Authority, with 1,400 employees, has in the past invested in real estate, cement, and banking in countries from Egypt to Sweden. Now it is becoming more active in finance.

Earlier this year, A.D.I.A. bought a small stake in Apollo Management, the New York-based private equity firm run by Leon Black.

Another Abu Dhabi government entity, Mubadala, owns a 7.5 percent stake in the Carlyle Group, while Istithmar, an arm of the Dubai government, recently bought Barneys New York.

The Dubai government-controlled Borse Dubai took Nasdaq's 28 percent stake in the London Stock Exchange off its hands not long ago, gaining a 19.9 percent share of Nasdaq in return.

These investments underscore the growing role that Middle Eastern investors are taking outside their home turf. Also yesterday, an investment company owned by the ruler of another United Arab Emirates principality, Dubai's Sheik Mohammed bin Rashid al-Maktoum, bought a stake in Sony.

The investment company, Dubai International Capital, a private equity firm, comes under the overall supervision of Dubai Holdings, Sheik Mohammed's umbrella organization for economic development, real estate projects, and media acquisitions. Dubai Holdings is headed by Mohammed Abdullah al-Gergawi, who holds the title of Dubai's minister of state for cabinet affairs, and who, far more significantly, is Sheik Mohammed's most trusted lieutenant and friend. Gergawi's wife, Mona al-Marri, is chief executive of Dubai's media conglomerate, Media Services Group. She plans to start a business daily that would be circulated around the world, as well as a speakers bureau.

Sheik Mohammed also holds the title of prime minister of the United Arab Emirates, but Dubai and Abu Dhabi regard each other as rivals, especially in the global arena, where Dubai's skills at public relations have obtained voluminous publicity for its construction, tourism, transportation, and financial-services projects.

Abu Dhabi is a more conservative emirate, although far wealthier than Dubai, whose modest oil reserves are expected to disappear in less than a decade. The Abu Dhabi Investment Authority manages much of Abu Dhabi's oil income, and reportedly has cash reserves of more than $1 trillion.

The head of the investment authority, Sheik Ahmed—one of 19 sons of the late Sheik Zayed, who helped create the U.A.E. in 1971 from what had been the British-administered Trucial States—is a friend and cousin of Sheik Nahayan. Moreover, Sheik Nahayan's father, Sheik Mabarak bin Mohammed al-Nahayan, served as an influential minister in Sheik Zayed's cabinet, and is widely regarded as one of the most revered figures in the Gulf.

In this part of the world, such relationships and reputations matter. Indeed, one of Sheik Nahayan's brothers leads Abu Dhabi's national carrier, Etihad Airways. One of Sheik Mohammed's brothers heads Dubai's highly successful Emirates Airline Group.

Manners and protocol also matter very much in this part of the world. Had Chuck Prince responded differently to the personal invitation from Sheik Nahayan, it still might not have been enough to save his job. But what was seen in Abu Dhabi as a breach of etiquette certainly raised a question in the minds of Persian Gulf decisionmakers about his leadership abilities.

The lesson for C.E.O.'s: When Abu Dhabi calls, respond immediately.