Heating the 1-alkenyl complexes cis-[Pt(1-alkenyl)2L 2] [alkenyl = 1-pentenyl, 1-hexenyl, 1-heptenyl, and 1-octenyl; L2 = 1,3-bis(diphenylphosphino)propane (dppp) or 1,2- bis(diphenylphosphino)ethane (dppe)] to 100°C caused a quantitative isomerization to the corresponding 2-alkenyl complexes cis-[Pt-(2-alkenyl) 2L2]. This novel isomerization is strongly dependent on the length of the alkenyl chains as well as the nature of the ligand, L 2. The new 2-alkenyl complexes were fully characterized by analytical and spectroscopic methods. An X-ray crystal structure of cis-[Pt(2-pentenyl) 2(dppp)] confirms this isomerization. Possible reaction mechanisms are discussed. © 2007 American Chemical Society.