Reactivity of neutral nitrogen donors in planar d8metal 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, k2f, 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, k2r, are very sensitive to the nature of the leaving group and plots of log k2r against the pKa, 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 pKa is linear with a slope of 0.66. The results are compared with data from the literature.