1 react regiospecifically in the presence of the thiotantalum(V) compound2 to afford the 1,2,4-thiadiphospholes 3. These compounds can function both as η1 and as η5 ligands in complexes with group-6 metals. [4+2] Cycloadditionreactions only occur with phosphaalkynes or highly reactive acetylenes as reaction partners. These reactions afford on the one hand the functionalized 1,2-thiaphospholes 17 and