FACTBOX - Ups and downs in U.S.-Iranian relations

(Reuters) - Iran's chief nuclear negotiator said he hoped for "good and constructive" talks when he left Tehran on Friday to meet world powers in Geneva on the country's disputed nuclear programme.

The United States is sending an envoy to the talks for the first time, seeking to underline to the Islamic Republic and others Washington's stated position that it wants a diplomatic solution to the impasse.

Here are some details of 55 years of turbulent relations between Iran and the United States.

1953 - A COUP

-- In August 1953, the CIA helped orchestrate the overthrow of Iran's popular Prime Minister Mohammed Mossadegh, restoring the Shah, Mohammed Reza Pahlavi, to power.

-- Washington acted after Britain, opposed to Mossadegh's policy of nationalising the British-controlled oil industry, convinced U.S. officials the prime minister was turning to communism. As Britain's power faded, the United States became the symbol of what many Iranians saw as Western imperialism.

1972 - A VISIT

-- A 1972 visit by U.S. President Richard Nixon cemented a close strategic relationship between Iran and the United States. Iran spent millions of petro-dollars buying some of the most advanced U.S. military equipment. But opposition to the Shah, led by exiled cleric Ayatollah Ruhollah Khomeini, mounted.

1979 - A REVOLUTION