PROTONOLYSIS OF DIALKYLARYLPLATINUM(II) 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, B u-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 on e 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 H-1 and P- 31 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 ste ric congestion at the Pt-C bond, varying from k(2) = (2.65 +/- 0.08) x 10(5) M-1 s(-1) for R = R' = Et to k(2) = 9.80 +/- 0.44 M(-1)s(-1) fo r R = R' = CH2Si(Me)(3). Low enthalpies of activation and largely nega tive volumes of activation are associated with the process. The mechan ism involves a rate-determining proton transfer either to the metal-ca rbon sigma bond (S(E)2 mechanism) or to the metal center (S-E(oxidativ e) mechanism), followed by fast extrusion of the alkane and simultaneo us 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 chara cterized by high values of enthalpies of activation, positive entropie s of activation, and largely positive volumes of activation. The react ion is shown to proceed through the dissociative loss of the weakly bo nded molecule of solvent and the interconversion of two geometrically distinct T-shaped 14-electron 3-coordinate intermediates. The presence of beta-hydrogens on the residual alkyl chain produces a great accele ration 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 interactio n. The results of this work, together with those of a previous paper, give a rationale of the ''elusive'' nature of these compounds. The fol lowing factors concur: (i) electron release by the phosphine ligands, (ii) steric repulsion and distortion of the square-planar configuratio n, and (iii) interaction of the metal with beta-hydrogens.