WASHINGTON — A former legislative aide who helped the FBI build a case against his Republican boss received probation and a fine but no jail time yesterday. A judge showed leniency for his work in convicting the congressman.

William Heaton let FBI agents record his telephone calls and taped a 2 1/2-hour meeting with a disgraced congressman, Bob Ney. Heaton leaked documents and worked late into the night and on weekends to avoid arousing suspicion that he was working with the government.

Federal prosecutors recommended that Heaton serve house arrest, but U.S. District Judge Ellen Segal Huvelle ordered him to serve two years probation and pay a $5,000 fine. Judge Huvelle said she would not have been so lenient had Heaton’s cooperation not been so exceptional.

“People don’t generally walk around congressmen wearing a wire,” Judge Huvelle said.

Heaton, 29, apologized several times in court. He said he was ashamed that he did not have the courage to stand up to Ney, who was trading political access for campaign donations, trips, and expensive gifts.

“American citizens should be able to trust those who work on their behalf,” he said in court. “I violated that trust.”