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.