Where was Richard Darman on the night of Wednesday, March 19? At home, in pain.

"Dick's back went bad -- he has a very bad back," Kathleen Darman, wife of the deputy secretary of the treasury, said yesterday. His absence at a dinner for Canadian Prime Minister Brian Mulroney apparently so unnerved hostess Sondra Gotlieb, sometime humor columnist and the wife of Canadian Ambassador Allan Gotlieb, that she slapped her social secretary in the face.

It happened reportedly after the victim, Connie Connor, informed the hostess that Darman -- an extra man, owing to his wife's plans to be out of town -- was not coming after all.

"I don't think anything is worth getting that upset about," the wife of Darman said.

"I must say," said Joan Braden, one of the guests, "you'd think that this particular dinner was of such importance that people wouldn't drop out. I would practically have to be in the hospital not to go."

She noted the dinner didn't lack for "major titles" -- as Gotlieb's literary creation, Popsie Tribble, might have put it -- including the vice president, the secretary of state and Darman's boss, Treasury Secretary James Baker, the president's former chief of staff who took his aide Darman with him when he left the White House. Kathleen Darman said it was after his arrival at Treasury that Darman's back troubles began.

But is he worth slapping one's social secretary over? "He's a good dinner guest, a very interesting conversationalist," said Anne Richardson, wife of Elliot, whose invitations Darman has accepted.

Darman -- who is well known for not going to Washington dinner parties -- was not returning phone calls yesterday.

But his absence Wednesday night should have come as no surprise to Sondra Gotlieb, who has been chronicling the vagaries of diplomatic society in her Washington Post column. As she wrote recently to her best friend Beverly, quoting Popsie Tribble, "Extra men can be unreliable.