1958, Was Iran's prime minister in the Soviet camp?

I'm guessing you're thinking of Mohammad Mossadegh, the prime minister who was overthrown in a 1953 coup? Mossadegh was not himself a communist, he was a liberal nationalist. However, he drew support from the communist Tudeh Party, which also favored nationalizing Iran's oil. The Tudeh party was growing in popularity and influence, and Mossadegh was increasingly dependent on their support. (Malcolm Byrne, "Mohammad Mosaddeq and the 1953 Coup in Iran.")

A declassified CIA history of the coup (Doc 3a here), written in the 1970s, shows that the CIA did not believe that Mossadegh himself was a communist, but he was willing to turn to the Soviet for help--and his policies were creating instability that would potentially lead to opportunities for the Soviet Union to expand its sphere of influence.

Had the British sent in the paratroops and warships, as they were to do a few years later against the Egyptians at Suez, it was almost certain that the Soviet Union would have occupied the northern portion of Iran.… It was also quite probable that the Soviet army would have moved south to drive British forces out on behalf of their Iranian “allies.” Then not only would Iran’s oil have been irretrievably lost to the West, but the defense chain around the Soviet Union which was part of U.S. foreign policy would have been breached. The Soviets would have had the opportunity to achieve the ancient Russian dream of a port on the Persian Gulf and to drive a wedge between Turkey and India. Under such circumstances, the danger of a third world war seemed very real. When it became apparent that many elements in Iran did not approve of Mosadeq’s continuing gamble or the direction in which he was pushing their country, the execution of a U.S.-assisted coup d’etat seemed a more desirable risk than letting matters run their unpredictable course. Mossadeq was already openly threatening to turn to other sources for economic help -- the Soviet Union -- if Britain did not meet his demands or if the United States did not come forth with massive aid to replace his lost oil revenue. Peacefully or in war, the Soviet Union appeared to be the only potential beneficiary of Mossadeq's policies.