From his spacious and elegant office on the first floor of a staid, old building in the heart of the Buenos Aires financial district, Miguel J. L. Rodriguez Diez manages one of the largest and least known empires in Argentina-one with a new connection to Riggs National Bank, the largest in metropolitan Washington.

Rodriguez Diez, his father-in-law, Hector Francisco Capozzolo, and other members of the Capozzolo family own every last share of the $100 million-plus holding company-called Arbol Solo-and run it as their private fiefdom. They do not have to-and do not-reveal any financial figures about their wealth or the net worth or profitability of any of their wholly owned companies.

Among other things, the Capozzolos, through Arbol Solo, own more of Argentina's rich pampas (360,000 hectares) and more cattle (200,000 head) than any other family or corporation in Argentina. With beef prices soaring and land values in Argentina skyrocketing almost as fast as the price of oil, it is almost impossible to put a value on the Capozzolos' agricultural interests.

But the land and cattle are only a part of the Capozzolo empire. Arbol Solo also owns Inversiones Unidas, which in turn owns Banco Tornquist, Argentina's 25th largest bank; FERRUM, a sanitary equipment manufacturer with 85 percent of the Argentine market; and the Compania Introductora de Buenos Aires, the country's major salt producer.

Inversiones Unidas also owns a metal goods manufacturing company called TAMET, two insurance companies, real estate in Buenos Aires, a large chunk of Diners' Club of Argentina "and parts of many other companies," Rodriguez Diez said in a recent interview.

The Capozzolos are already probably among the world's 100 wealthiest families, which would make them interesting all by itself. But there is another reason for Washingtonians to be interested in them.

The Capozzolos and partner, Jorge E. Carnicero, recently have become the largest stockholder in Washington's largest bank, Riggs, having purchased more than 10 percent of its outstanding stock. According to Rodrigeuz Diez and Carnicero, they are planning to buy more if the bank continues to be profitable and if Riggs' stock continues to be reasonably priced.

Rodriguez Diez said the Capozzolos are interested in Riggs "exclusively as an investment," that they began buying into the bank because they were convinced its present management is sound and that they do not want nominal or actual control of the financial institution.

Essentially, he said, the decision to by into Riggs was part of a longer-term plan begun about six years ago to diversify the family's investments away from their strictly agrarian interests into manufacturing and banking-first in Argentina and now in the United States and possibly Europe. $"We were looking for a very well-run bank (in the United States) with a good management team" because "we thought it would be interesting to take a position in such a bank," Rodriguez Diez said. It was Carnicero, an Argentine-born U.S. citizen who is chairman and founder of the McLean-based Dynalectron Corp., who brought Riggs to the Capozzolos' attention about a year ago.

Carnicero had owned shares in Riggs for 15 years and was a director of the bank when he suggested to the Capozzolos that they form a partnership with him to buy a substantial portion of Riggs' outstanding stock.

Over the past eight months, the partners have brought, or control for voting purposes, more than 252,000 Riggs shares. Carnicero and two of his nominees now sit on the Riggs board.

Both Rodriguez Diez, who, among his many other titles, is president of the Banco Tornquist, and Leonardo Anidjar, who actually runs the bank, denied that the Capozzolos have any intention of merging Banco Tornquist into Riggs or combining the two banks in any way to increase their international clout.

"It has never occured to us," Anidjar said, adding that he expects nothing more than to keep the traditional correspondent banking relationship between Riggs and Banco Tornquist that existed before the Capozzolos took their position in the Washington bank, which with $2.4 billion in assets, is considered a medium-sized bank by U.S. standards.

Both the Capozzolos and Banco Tornquist have longstanding relationships with far larger U.S. banks, according to Rodriguez Diez, Anidjar and sources within the American banking community in Buenos Aires.

Citibank of New York has long been a source of funds for Arbol Solo and the Capozzolos, while Banco Tornquist has lines of credit with such major U.S. banks as Chase Manhattan, Bankers Trust and Wells Fargo. The Banco Tornquist is not even Arbol Solo's main bank in Argentina, Anidjar said. Indeed, Rodriguez Diez said, it was Citibank that first brought Carnicero and the Capozzolos together six years ago and helped put together the deal that first took the Capozzolos into the manufacturing and financial sectors in Argentina.

In 1973, the Capozzolos had arranged financing through Citibank's Argentine representative to buy a bank in Buenos Aires but the deal fell through because of "bureaucratic delays," Rodriguez Diez said. "But one day, a little later, Citibank created a new department looking for new businesses to buy," and came to Arbol Solo with a proposition.

Carnicero; Alejandro Shaw, president of Banco Shaw, a very large financial institution in Buenos Aires; and Baron Van de Put of Belgium had purchased 51 percent of the troubled Ernesto Tornquist Y Cia, a holding company that owned the Banco Tornquist, FERRUM, TAMET and Introductora de Buenos Aires, the salt company, among other assets.

The three partners were looking for an Argentine company to buy some of their stock and manage Ernesto Tornquist. Carnicero had asked Citibank to look for an Argentine partner, and Citibank found the Capozzolos.

The Capozzolos initially bought two-thirds of the partnership's percentage and then, in June or July of 1975, bought the remaining one-third of Ernesto Tornquist owned by Carnicero, Shaw and the Baron. By 1977, Arbol Solo and the Capozzolos had purchased 98 percent of the stock in Tornquist Y Cia, Rodriguez Diez said, and thus gained complete control of the companies in the Tornquist group.

It was about that time the Capozzolos changed the name of Ernesto Tornquist Y Cia to Invesiones Unidas.

Although the Capozzolos and Carnicero had not known each other before Citibank brought them together, Rodriguez said by the end of 1975 "we had a very cordial relationship with Mr. Carnicero," who from time to time would bring various business propositions to Buenos Aires for the Capozzolos to consider.

"We analyzed some other businesses to buy together" but nothing seemed right until about a year ago, when Carnicero suggested that Riggs National Bank might make an interesting investment. The Capozzolos decided it would.

In Argentina, there are nasty rumors about almost every businessman, from the neighborhood grocer to the country's richest families. The Capozzolos have not entirely escaped this syndrome, having been linked with every boogeyman from the Peronist guerrilas to the Mafia, largely because they seem to have made so much money so quickly.

In the tradition of the Rothschilds, who made their great fortune by learning before anyone else that the British had defeated Napoleon at Waterloo, the Capozzolos bought cheap and held on, gambling that land, cattle and other real assets would always be worth something even in the worst of times-and increase dramatically when things got better, as they are now beginning to in Argentina.

By the mid-1970s, the Capozzolos had made so much money that they were in a position to go one step further-buying companies or stock outside of their native country. They have amassed their fortune quietly and are hardly known in Argentina's social world, which revolves around the exclusive Jockey Club, the summer resort of Punta del Este, Uruguay, and the great houses in Palermo Chico, the most fashionable section of Buenos Aires.

Until their purchase of the Riggs stock, which because of U.S. banking laws is a matter of public record, one of the world's richest families had remained almost unnoticed.

They obviously do not much enjoy the newfound attention, but Rodriguez Diez said he does not think the fact that the Capozzolos have taken a position in Riggs will affect either his family in Argentina or cause resentment among the bank's Washington customers. CAPTION: Picture 1, Banco Tornquist in Buenos Aires: part of a $100 million-plus holding company. AP; Picture 2, Hector Francisco Capozzolo: a company run like a private fiefdom.; Picture 3, Miguel J. L. Rodriguez Diez: no worried about an adverse reaction. AP photos