η<sup>2</sup>‑Allenyl- and η<sup>2</sup>‑Alkynylphosphonium Complexes of Platinum
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The reaction of the propargylic salt [Ph3PCH2CCH]PF6 with [Pt(C2H4)(PPh3)2] affords the η2-allenylphosphonium salt [Pt(η2-CH2CCHPPh3)(PPh3)2]PF6 via a metal-mediated propargylic rearrangement. Isomerization of the complex occurs in solution to generate the salt [Pt{η2-C(PPh3)CCH2}(PPh3)2]PF6, wherein the allene is coordinated by the internal CC bond. Computational studies indicate that the isomerization of the propargylic cation [Ph3PCH2CCH]+ to the allenyl species [Ph3PCHCCH2]+ and the α-alkynyl isomer [Ph3PCCCH3]+ is thermodynamically favorable. The isomeric η2-alkynylphosphonium salt [Pt{η2-C(CH3)CPPh3}(PPh3)2]PF6 is formed from the reaction of [Ph3PCCCH3]PF6 with [Pt(C2H4)(PPh3)2] and does not isomerize to the propargylic or allenyl forms.
