Mr. Suleman will also resign from his post as executive vice president and chief financial officer of radio syndicator Westwood One, which is minority-owned by Viacom and managed by Infinity.
Citadel, which is owned by Forstmann Little, owns and operates radio stations in mid-sized markets.
Mel Karmazin, Viacom's president and chief operating officer, will assume Mr. Suleman's duties at Infinity while searching for a permanent replacement.
Westwood One also is looking for a new CFO, but the two posts will not necessarily be filled by the same person. Mr. Suleman will remain on the board of directors at Westwood One.