A Cationic Unsaturated Platinum(II) Complex that Promotes the Tautomerization of Acetylene to Vinylidene

Complex [PtMe₂ (PMe₂ ArDipp2 )] (1), which contains a tethered terphenyl phosphine (ArDipp2 =2,6-(2,6-ⁱ Pr₂ C₆ H₃ )₂ C₆ H₃ ), reacts with [H(Et₂ O)₂ ]BAr_F (BAr_F^- =B[3,5-(CF₃ )₂ C₆ H₃ ]₄^- ) to give the solvent (S) complex [PtMe(S)(PMe₂ ArDipp2 )]⁺ (2⋅S). Although the solvent molecule is easily displaced by a Lewis base (e.g., CO or C₂ H₄ ) to afford the corresponding adducts, treatment of 2⋅S with C₂ H₂ yielded instead the allyl complex [Pt(η³ -C₃ H₅ )(PMe₂ ArDipp2 )]⁺ (6) via the alkyne intermediate [PtMe(η² -C₂ H₂ )(PMe₂ ArDipp2 )]⁺ (5). Deuteration experiments with C₂ D₂ , and kinetic and theoretical investigations demonstrated that the conversion of 5 into 6 involves a Pt^II -promoted HC≡CH to :C=CH₂ tautomerization in preference over acetylene migratory insertion into the Pt-Me bond.