Reactivity of neutral nitrogen donors in planar d8 metal complexes. Part 2. The system [2,6-bis(methylsulfanyl-methyl)pyridine]chloroplatinum(II) with pyridines and amines in methanol. Effect of basicity, π-acceptor capacity and steric hindrance

Abstract

The kinetics of the forward and reverse steps of the process [Pt(C₅H₃N(CH₂SMe)₂}Cl]⁺+ am ⇌[Pt{C₅H₃N(CH₂SMe)₂}(am)]²⁺+ Cl^–[C₅H₃N(CH₂SMe)₂= 2,6-bis(methylsulfanylmethyl)pyridine; am = one of a number of pyridines and some heterocyclic and aromatic amines covering a wide range of basicity] has been studied in methanol at 25°C. Both forward and reverse reactions obey the usual two-term rate law observed in square-planar substitution. The second-order rate constants for the forward reactions, k₂^f, show only a slight dependence upon the basicity of the entering am, and steric hindrance markedly decreases the reactivity. The second-order rate constants for the reverse reactions, k₂^r, are very sensitive to the nature of the leaving group and plots of log k₂^r against the pK_a, of the conjugate acids of unhindered pyridines with different π systems are linear with a slope of –0.45. A comparison among the different pyridines and sp³ nitrogen donor bases indicates an appreciable π contribution to the stability of the Pt–N(sp²) bond. The equilibrium constants for the reactions have been determined in a number of cases from the ratio of the rate constants and a plot of log K against the pK_a is linear with a slope of 0.66. The results are compared with data from the literature.