After Mexico stunned the international financial world last August by announcing it was unable to pay its $80 billion of foreign debt on time, Citibank senior officer William R. Rhodes emerged as the most important banker in the massive effort to renegotiate Mexico's huge outstanding loans.

In short order last winter, as one after another nation found itself over its head in debt, Bill Rhodes, at age 48, became the most important foreign banker in Latin America.

Rhodes also chaired the bank advisory groups charged with negotiating new loan agreements with Argentina, Peru and Uruguay.

His leadership, ability to work out deals, command of detail, knowledge of the countries involved and their leaders and his constant attention to communicating with big and small banks were instrumental in producing new loan packages for all four nations, his colleagues said.

He was the intermediary between the banks and the International Monetary Fund, between the banks and the debtor country and the simultaneous peacemaker and prodder of the banks--themselves often deeply divided.

"I can remember him sitting there in meetings with the IMF on Mexico playing with his cigar, one of those long thin ones, and being from the bankers' side very much in control," said a Reagan administration official, who claims Rhodes "husbanded, fathered and mothered" the Mexican deal.

Rhodes, while hardly a household name here, is well known and respected in the international banking community and in Latin America. He has earned the reputation as something of a superman in the arcane art of restructuring loans for debt-laden countries.

His reputation was further enhanced when the rescue package negotiated last winter between Brazil and its lenders fell apart. Brazil was the only major negotiation in which Rhodes played no role.

Last month, with the desperation of an oil driller calling for Red Adair to extinguish a major oil well blaze, Brazil sent for Bill Rhodes to head up its bank advisory committee. Not everyone thinks Rhodes is a superman, but even his critics think Rhodes would have handled the Brazilian negotiations better.

"If nothing else, he probably would have talked some sense into the Brazilians," said a major banker.

The initial negotiations were handled by a Citibank colleague--Gerard Finneran--and a top official at Morgan Guaranty Trust Co., Antonio Gebauer. Neither has yet played a role in Phase II of the Brazilian debt negotiations.

Soft-spoken and bespectacled, Rhodes looks more the Clark Kent side of his Superman image. But colleagues vouch for his Superman-like stamina.

A jogger, Rhodes laments the difficulty in finding time to take his daily run around the reservoir in Manhattan's Central Park. His last real vacation was interrupted by the advent of the Mexican financial crisis. When he is not chairing one or another meeting of major banks, he is working the telephone in his modest office at Citibank's headquarters on Park Avenue. He picked up the cigar habit he once had dropped.

Rhodes has been a Latin American specialist at Citibank, New York's largest, since he graduated from Brown University in 1957. Over the years, Rhodes has gotten to know most of the important political and economic personalities in Central and South America. He talks about them with seeming instant recall. Rhodes has served in Jamaica and Venezuela and headed Citibank's operations in Latin America and the Caribbean.

Since 1980, Rhodes has been Citibank's senior corporate officer for Latin America, the Caribbean and sub-Sahara Africa.

It's appropriate that Rhodes is now in charge of many of the emergency loan negotiations for Latin American nations that cannot meet their huge debt payments on time, said one of his Central American acquaintances. Rhodes, he said, played a major role in making the loans that now must be restructured. He served in key bank positions in Latin America when Citibank, as well as most other major banks in the world, spent most of their time "recycling" to oil-consuming countries the huge profits earned by oil producers in the 1970s and early 1980s.

"He was good at lending money. Where he'll come unstuck is if these loans come unstuck," the Central American said.

This person said he thought Rhodes would operate in debt renegotiations as he believed he had in the original deals--with his eye on making profits for the banks. "He is probably an extraordinarily successful guy because he just goes after things--he doesn't get diverted by philosophy or substantive discussion" about development issues.

A Washington official described Rhodes as probably more of a "deal-maker" than a thinker. He is "clever . . . and very, very good at what he does," another person said, but probably "not profound."

In 1979 and 1980, Rhodes got his feet wet in the restructuring game, heading up the bank committees working out loans to Jamaica and Nicaragua.

He said the effort in Nicaragua got an unexpected assist from Fidel Castro. In Managua during debt negotiations, Rhodes was summoned by Castro at a cocktail party celebrating the first anniversary of the Sandinista takeover. Castro, he said, told him that Cuba had made a mistake breaking off financial relationships with the rest of the world and had advised Nicaragua to honor its foreign debts.

He said Castro promised him a box of cigars when the Nicarguan negotiations were finished. Nicaragua signed a restructuring agreement shortly thereafter. Castro never sent the cigars, Rhodes said.

Rhodes not only has met most of the key actors in Latin America, he talks about them as if he knows them intimately. He has a phemonenal memory for names, said one acquaintance. He is also famous for dropping those names in nearly every conversation. But part of Rhodes' effectiveness, the acquaintance said, is his ability to know, or appear to know, all the important players.

Moreover, in a profession whose practitioners tend to be regarded frostily by outsiders, Rhodes appears to be well-liked.

"He remembers people's names, greets them warmly . . . he would not get irritated, or if he gets irritated he wouldn't show it," said one colleague of Rhodes.

Working out the emergency loan packages needed in the wake of the Third World debt crisis involves close coordination with Western governments as well as with banks and international agencies. Rhodes is now a frequent caller at the Treasury Department in Washington and the Federal Reserve Board as well as at the IMF, one Reagan official said. "Probably someone in Treasury talks to him two or three times a week, and the same in the Fed," he said. Rhodes is also in close contact with top officials at the Bank of England and in the British treasury department.

Rhodes does seem to know everyone, said the head of Latin American lending at a major U.S. bank, "but you can't ascribe it totally to Bill's personality, although that's a large part of it. If Citibank sent Howdy Doody to head its Brazil operations, he would have access to the presidential palace."

Backing up Rhodes in the negotiating process is Citibank's vast, sophisticated network of communications equipment. Several hundred banks at the minimum were involved in each loan restructuring. In Mexico's case, about 1,300 banks had to be advised of and approve the negotiations. Telexes poured forth from Citibank every day.

Although the loans are talked of in the aggregate billions of dollars, in fact each restructuring involved hundreds of individual loans. Some involved only a few banks and were measured in tens rather than hundreds of millions of dollars. Each loan had to be worked out individually. For next year--every country that had to reschedule its 1983 debt will have to do the same in 1984--Rhodes has devised a more standardized format for the Mexican loans that come due. If the approach works, much of the tedious paperwork may be eliminated.

Rhodes' and Citibank's ability to manage the paperwork in the refinancing go a long way toward explaining the seeming success in the Mexican and Argentinian restructurings--although Rhodes cautions that it will be some time before most troubled countries will be out of the woods.

But Rhodes' style itself is as crucial as the back-up Citibank provides, bankers said.

Bankers never seem to agree on anything, said one New York banker. Rhodes is good at getting the consensus needed.

"He has an incredible ability to make you feel important and part of the process while at the same time saying basically nothing himself," said a banker who has watched Rhodes in the negotiating process. "But he's a good professional. He knows when to push and when to sit back."

Another colleague described Rhodes as "very energetic, very dynamic . . . a very pushy, persistent guy." When he arranges something, he will follow it up and make sure things get done, he said. He works the phone a lot, and is careful to bring everybody on board.

At a recent meeting about Brazil in Chicago, Rhodes was careful to hear out the positions of all the banks in the newly formed coordinating committee--designed to bring in the views of the regional bankers who felt their needs were ignored during the negotiations that led to the now-failed Phase I agreement on Brazil last winter.

He listened to all points and assured the banks that nothing had been set in granite by the major New York banks that still comprise the advisory committee that must make the final decisions on Phase II. "Even if you eventually lose the point, it helps to know it was considered," said one banker who was in Chicago.

A U.S. government official said, "from our point of view he doesn't come on too strong." Instead of making money demands on the government, Rhodes will discuss more analytically and coolly what needs to be done to keep a country solvent, this official said.

But Phase II of Brazil may be too much even for Rhodes, and even if he presides in a Superman costume.

The 800 or so banks that are lenders to Brazil are sharply divided among themselves. The smaller ones want out. The big banks are wary that Brazil lacks the political will to make hard economic decisions. Brazil itself is upset with the banks.

"Brazil felt it had played the game by the banks' rules," said W. Denis Wright, chief of Latin American lending at Continental Illinois National Bank. Brazil had never negotiated the razor-thin interest margins demanded by some other Latin American borrowers and, unlike Mexico, had continued to honor its banking commitments--including loan repayments--during its protracted negotiations last year.

But the smaller banks, Wright noted, treated Brazil as if it were Mexico. The banks were locked into Mexico, which declined to pay anything but interest on its debts and would not repay maturing deposits. The smaller banks got out of Brazil. The big banks demanded high fees and interest rates for the new loans.

Brazil has been cut off by the banks and the IMF since May. Until the funds start flowing again--Brazil appears finally to have reached a new pact with the IMF--the country will fall ever deeper in arrears.

With the banks and the IMF skeptical, with the smaller banks still dying to get out of their remaining Brazilian commitments and with Brazil itself angered by the treatment it has received and fearful that further austerity could cause severe social upheavals, Phase II of the Brazilian negotiations will be the toughest assignment by far that Rhodes has faced.

"It's Mission Impossible," said a major banker.