BOSTON – William Nunn Lipscomb Jr., a Harvard University professor who won the Nobel chemistry prize in 1976 for his work on man-made compounds consisting of boron and hydrogen and the problems of chemical bonding, has died, his son said Friday. He was 91.

Lipscomb died Thursday night at Mount Auburn Hospital in Cambridge, Mass., of pneumonia and complications from a fall, said his son, James Lipscomb.

Two of Lipscomb’s graduate students and a third who spent time at his lab have won Nobels. Yale University professor Thomas Steitz, who shared the 2009 chemistry prize, said Lipscomb was an inspiring teacher who encouraged creative thinking.

“He was a great mentor, letting us work freely, yet continually putting before us puzzles to be explained,” said Lipscomb’s first graduate student at Harvard, Roald Hoffman, who was awarded the chemistry prize in 1981.

Lipscomb graduated from the University of Kentucky.