Complex sentence: Niven had a long, complex relationship with Goldwyn, who gave him his first start, but the dispute over "The Elusive Pimpernel" and Niven's demands for more money led to a long estrangement between the two in the 1950s.
Split into simpler sentences: Niven had a long, complex relationship with Goldwyn, who gave him his first start. But the dispute over "The Elusive Pimpernel" and Niven's demands for more money led to a long estrangement between the two in the 1950s.