The substitution reactions of sulfide by phosphines on Pt(IV) complexes having a cyclometalated imine ligand, two methyl groups in a cis geometrical arrangement, and a halogen and a sulfide as ligands, [Pt (Me)(2)X(CN)(SR2)], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines, sulfides, X-1 and CN. In most of these cases, a limiting dissociative mechanism has been found, where the dissociation of the sulfide ligand corresponds to the rate-determining step. The intermediate species formed behaves as a true pentacoordinated Pt(IV) compound in a steady-state concentration only for the systems with SMe2; for the bulkier SEt2 and SBzl(2) leaving ligands the rate constants and activation parameters show an important degree of solvent dependence, which correlates with the ability of the solvent to form hydrogen bonds, The X-ray crystal structure of one of the dibenzyl sulfide complexes has been determined, and the geometrical arrangement of the ligands has been determined by NOE NMR measurements at low temperature. The nature of the solvent, imine, sulfide, and halogen ligands produces differences in the reaction rates, which can be quantified very well by the corresponding AS' values that move from +48 to -90 J K-1 mol(-1). The reaction on [Pt(Me)(2)F(C-5(C) under barF(4)CH (N) under bar CH2Ph) (SME2)] has been found to take place via a mechanism that depends strongly on the bulkiness of the substituting phosphine, While for PCy3 the reaction is dissociative, for smaller entering ligands the first associatively activated substitution mechanisms on organometallic Pt(IV) complexes have been established with values of DeltaH(double dagger) and DeltaS(double dagger) in the 28-44 kJ mol(-1) and -120 to -83 J K-1 mol(-1) ranges. Important intramolecular hydrogen bonding in the starting material can be held responsible for this difference with the remaining systems.