The reversible oxidative addition reactions of methyl(halogeno)tin and methyl(halogeno)germanium compounds to electron-rich platinum(II) complexes of the type [PtMe2(diimine)] have been studied. Complete kinetic and thermodynamic parameters have been obtained by VT H-1 NMR for the reversible oxidative addition of Me3EX (E = Sn, X = Cl, Br, I) to [PtMe2(bpy-(t)bu(2))] (bpy-(t)bu(2) = 4,4'-di-tert-butyl-2,2'-bipyridyl) and related compounds, while partial data have been obtained for the reductive elimination of Me2SnCl2 from [PtClMe2(Me2SnCl)-(bpy-(t)bu(2))] and for the oxidative addition of Me3GeCl to [PtMe2(bpy-(t)bu(2))]. W-visible spectroscopic studies have also yielded equilibrium constants and Delta G degrees for the reversible oxidative addition reactions of Me3SnX (X = Cl, Br, I) to [PtMe2(diimine)]. Thermodynamic studies quantitatively establish the halogen effect on the oxidative addition reactions studied according to the favorability series I > Br > Cl. Kinetic studies clearly point to an S(N)2 mechanism for the reactions studied, and this is further supported by the observation of the cationic complex [PtMe2(Me3Sn)-(bpy-(t)bu(2)){OC(CD3)(2)}](+) at low temperature in acetone-d(6). Extremely large second-order rate constants are observed for the oxidative addition of Sn-X bonds to dimethylplatinum(II) complexes, some being greater than 10(8) M-1 s(-1), and it is established that rates follow the series Sn > Ge > Si > C and I > Pr > Cl. Estimates have been made of the Pt-MMe3 bond dissociation energies for [PtXMe2(MMe3)(bpy-(t)bu(2))], X = halide, and these are 233, 182, and 172 kJ mol(-1) for M = Si, Ge, and Sn, respectively; the values are useful in rationalizing the chemistry of the Pt-M bonds.