Protonolysis of Dialkyl- and Alkylarylplatinum(II) Complexes and Geometrical Isomerization of the Derived Monoorgano−Solvento Complexes:  Clear-Cut Examples of Associative and Dissociative Pathways in Platinum(II) Chemistry

Protonolysis of the complexes cis-[PtR2(PEt3)2] (R = Me, Et, Pr n , Bu n , CH2C(Me)3, CH2Si(Me)3) and cis-[Pt(R)(R‘)(PEt3)2] (R = Ph, 2-MeC6H4, 2,4,6-Me3C6H2; R‘ = Me) in methanol selectively cleaves one alkyl group, yielding cis-[Pt(R)(PEt3)2(MeOH)]+ and alkanes. The reactions occur as single-stage conversions from the substrate to the product. There is no evidence by UV and by low-temperature 1H and 31P NMR spectroscopy for the presence of significant amounts of Pt(II) or Pt(IV) intermediate species. Reactions are first order with respect to complex and proton concentrations and are strongly retarded by steric congestion at the Pt−C bond, varying from k 2 = (2.65 ± 0.08) × 105 M-1 s-1 for R = R‘ = Et to k 2 = 9.80 ± 0.44 M-1s-1 for R = R‘= CH2Si(Me)3. Low enthalpies of activation and largely negative volumes of activation are associated with the process. The mechanism involves a rate-determining proton transfer either to the metal−carbon σ bond (SE2 mechanism) or to the metal center (SE(oxidative) mechanism), followed by fast extrusion of the alkane and simultaneous blocking of the vacant coordination site by the solvent to generate cis-[Pt(R)(PEt3)2(MeOH)]+ species. The subsequent slower process, cis to trans isomerization of cis-[Pt(R)(PEt3)2(MeOH)]+, is characterized by high values of enthalpies of activation, positive entropies of activation, and largely positive volumes of activation. The reaction is shown to proceed through the dissociative loss of the weakly bonded molecule of solvent and the interconversion of two geometrically distinct T-shaped 14-electron 3-coordinate intermediates. The presence of β-hydrogens on the residual alkyl chain produces a great acceleration of the rate (R = Me, k i = 0.0026 s-1; R = Et, k i = 44.9 s-1) as a consequence of the stabilization of the 3-coordinate [Pt(R)(PEt3)2]+ transition state through an incipient agostic interaction. The results of this work, together with those of a previous paper, give a rationale of the “elusive” nature of these compounds. The following factors concur: (i) electron release by the phosphine ligands, (ii) steric repulsion and distortion of the square-planar configuration, and (iii) interaction of the metal with β-hydrogens.