Kinetic study of the ligand substitution on the trimethylplatinum(IV) complexes with unidentate ligands

Ligand substitution kinetics for the reaction [Pt^IVMe₃(X)(NN)] + NaY = [Pt^IVMe₃(Y)(NN)] + NaX, where NN = bipy or phen, X = MeO^-, CH₃COO^-, or HCOO^-, and Y = SCN^- or N₃^-, has been studied in methanol at various temperatures. The kinetic parameters for the reaction are as follows. The reaction of [PtMe₃(OMe)(phen)] with NaSCN: k₁ = 36.1 ± 10.0 s^-1; ΔH₁^≠ = 65.9 ± 14.2 kj mol^-1; ΔS₁^≠ = 6 ± 47 J mol^-1 K^-1; k₂ = 0.0355 ± 0.0034 s^-1; ΔH_-2^≠ = 63.8 ± 1.1 kJ mol^-1; ΔS_-2^≠ = -58.8 ± 3.6 J mol^-1 K^-1; and k_-1/k₂ = 148 ± 19. The reaction of [PtMe₃(OAc)(bipy)] with NaN₃: k₁ = 26.2 ± 0.1 s^-1; ΔH₁^≠ = 60.5 ± 6.6 kJ mol^-1; ΔS₁^≠ = -14 ± 22 J mol^-1K^-1; k_-2 = 0.134 ± 0.081 s^-1; ΔH_-2^≠ = 74.1 ± 24.3 kJ mol^-1; ΔS_-2^≠ = -10 ± 82 J mol^-1K^-1; and k_-1/k₂ = 0.479 ± 0.012. The reaction of [PtMe₃(OAc)(bipy)] with NaSCN; k₁ = 26.4 ± 0.3 s^-1; ΔH₁^≠ = 59.6 ± 6.7 kJ mol^-1; ΔS₁^≠ = -17 ± 23 J mol^-1K^-1; k_-2 = 0.174 ± 0.200 s^-1; ΔH_-2^≠ = 62.7 ± 10.3 kJ mol^-1; ΔS_-2^≠ = -48 ± 35 J mol^-1K^-1 and k_-1/k₂ = 1.01 ± 0.08. The reaction of [PtMe₃(OOCH)(bipy)] with NaN₃: k₁ = 36.8 ± 0.3 s^-1; ΔH₁^≠ = 66.4 ± 4.7 kJ mol^-1; ΔS₁^≠ = 7 ± 16 J mol^-1K^-1; k_-2 = 0.164 ± 0.076 s^-1; ΔH_-2^≠ = 47.0 ± 18.1 kJ mol^-1; ΔS_-2^≠ = -101 ± 61 J mol^-1 K^-1; and k_-1/k₂ = 5.90 ± 0.18. The reaction of [PtMe₃(OOCH)(bipy)] with NaSCN: k₁ = 33.5 ± 0.2 s^-1; ΔH₁^≠ = 58.0 ± 0.4 kJ mol^-1; ΔS₁^≠ = -20.5 ± 1.6 J mol^-1 K^-1; k_-2 = 0.222 ± 0.083 s^-1; ΔH_-2^≠ = 54.9 ± 6.3 kJ mol^-1; ΔS_-2^≠ = -73.0 ± 21.3 J mol^-1 K^-1; and k_-1/k₂ = 12.0 ± 0.3. Conditional pseudo-first-order rate constant k₀ increased linearly with the concentration of NaY, while it decreased drastically with the concentration of NaX. Some plausible mechanisms were examined, and the following mechanism was proposed.>