The portrait of the founder of the Morgan Library can be found in the new Morgan Dining Room, which served as the family dining room at the turn of the century. It depicts John Pierpont Morgan as a young boy.

And it was that boy’s spirit that filled the building Tuesday night at the first black-tie celebration of the library’s expansion, which also marked the library’s centennial.

Eyes widened and fingers pointed upward to the walls of glass and the unveiled facades of the original buildings. Some jumped up and down in the all-glass elevators, just as the children of construction workers had done at their celebration Friday.

Yes, this was an exciting evening for the trustees and major donors of the library, an august crowd including Annette de la Renta, James Houghton, and three descendants of Morgan: Robert Pennoyer, Charles Morgan, and John Morgan.

“This is really our first night here,” the president of the library, Parker Gilbert, said. “I feel great. Thank goodness it’s done.”

The director of the museum, Charles Pierce, expressed his affection for the architect Renzo Piano’s work: “He has suffused the entire design with a light and transparency that is truly extraordinary,” Mr. Pierce said.

And the 200 guests there to see it agreed. “It’s a stunning success,” the city’s landmarks commissioner, Robert Tierney, said. “Few could manage this modern intervention in a historic complex. Renzo has, with his sense of balance and urbanism.”

It seemed like a major moment for New York. “I’m overwhelmed,” a former museum leader, Samuel Miller, said. I’ve known the building since I came to New York in the ’50s. I’ve always loved it, but what’s happened is a transformation.”

What would Morgan have thought? “His tastes ran more to the old,” the noted literary chronicler of New York society, Louis Auchincloss, who wrote a book about Morgan’s collecting interests, said.

The editor of Vogue, Anna Wintour, the heads of the Metropolitan Museum of Art, Philippe de Montebello and Emily Rafferty, and the director of the Frick Collection, Anne Poulet, to name just a few of the cultural and business leaders gathered, spent the cocktail hour exploring the new architecture and revisiting their favorite works, from illuminated manuscripts to Mozart compositions to a letter written by Bob Dylan. And then it was on to dinner in the four-story court, in the green glow of ficus and bamboo trees.

Mr. Pierce first welcomed guests. Then came brief words from Mr. Gilbert; the city’s cultural affairs commissioner, Kate Levin; Mr. Piano; the chairman of JPMorgan Chase’s board, William Harrison Jr., and the co-president of Morgan Stanley, Zoe Cruz. (JPMorgan Chase and Morgan Stanley provided funding for the dinner.) Mayor Bloomberg came to give his congratulations yesterday morning.

The menu consisted of seared and smoked salmon, chilled cucumber soup, filet mignon with pinot noir sauce, potatoes, and asparagus.

With several museums in New York undergoing expansions, a standard is being set for the first celebratory dessert – namely, that it evoke the architecture of the building. On this night, a coconut lemon cake was decorated with miniature representations of the library’s facades made of hand painted white chocolate.

agordon@nysun.com