η²‑Allenyl- and η²‑Alkynylphosphonium Complexes of Platinum

The reaction of the propargylic salt [Ph₃PCH₂CCH]­PF₆ with [Pt­(C₂H₄)­(PPh₃)₂] affords the η²-allenylphosphonium salt [Pt­(η²-CH₂CCHPPh₃)­(PPh₃)₂]­PF₆ via a metal-mediated propargylic rearrangement. Isomerization of the complex occurs in solution to generate the salt [Pt­{η²-C­(PPh₃)CCH₂}­(PPh₃)₂]­PF₆, wherein the allene is coordinated by the internal CC bond. Computational studies indicate that the isomerization of the propargylic cation [Ph₃PCH₂CCH]⁺ to the allenyl species [Ph₃PCHCCH₂]⁺ and the α-alkynyl isomer [Ph₃PCCCH₃]⁺ is thermodynamically favorable. The isomeric η²-alkynylphosphonium salt [Pt­{η²-C­(CH₃)CPPh₃}­(PPh₃)₂]­PF₆ is formed from the reaction of [Ph₃PCCCH₃]­PF₆ with [Pt­(C₂H₄)­(PPh₃)₂] and does not isomerize to the propargylic or allenyl forms.