AbstractComplex [PtMe2(PMe2Ar )] (1), which contains a tethered terphenyl phosphine (Ar =2,6‐(2,6‐iPr2C6H3)2C6H3), reacts with [H(Et2O)2]BArF (BArF−=B[3,5‐(CF3)2C6H3]4−) to give the solvent (S) complex [PtMe(S)(PMe2Ar )]+ (2⋅S). Although the solvent molecule is easily displaced by a Lewis base (e.g., CO or C2H4) to afford the corresponding adducts, treatment of 2⋅S with C2H2 yielded instead the allyl complex [Pt(η3‐C3H5)(PMe2Ar )]+ (6) via the alkyne intermediate [PtMe(η2‐C2H2)(PMe2Ar )]+ (5). Deuteration experiments with C2D2, and kinetic and theoretical investigations demonstrated that the conversion of 5 into 6 involves a PtII‐promoted HC≡CH to :C=CH2 tautomerization in preference over acetylene migratory insertion into the Pt−Me bond.