Displacement of neutral nitrogen donors by chloride in [Pt(SNS)(R-py)](ClO4)2 (SNS = 2,6-bis(methylthiomethyl)pyridine; R-py = pyridines): A reactivity comparison between meta- and para-substituted pyridines

The kinetics of the process Pt(SNS)(R-py)]²⁺ + Cl⁻ → Pt(SNS)Cl]⁺ + R-py {SNS = 2,6-bis(methylsulfanylmethyl)pyridine; R-py = meta- or para-substituted pyridines covering a wide range of basicity} were studied in methanol at 25 °C. The reactions obey the usual two-term rate law observed in the substitution reactions of square-planar d⁸ complexes. The plots of log k₂ {k₂ = second-order rate constants} against the pK_a of the heterocycles conjugate acids highlighted a different sensitivity of the two groups of N-donors to changes in basicity, thepara-substituted pyridines (4R-py) showing a weaker dependence on pK_a than the meta-substituted (3R-py). The results have been explained on the basis of a π-acidity difference between 3R-py and 4R-py which influences the reaction ground state.