1a–c are shown to very efficiently catalyze the cycloisomerization of alkynoicacids into alkylidenelactones via metal–ligand cooperation (TON up to 2000). Complexes 1a–c are competent toward a broad range of alkynoicacids, including functionalized and internal ones, and give access to 5- as well as 6- and 7-membered lactones in excellent yields and with very high selectivities.