Reactivity of neutral nitrogen donors in planar d8 metal complexes. Part 1. The system [1,2-bis(phenylsulfanyl)-ethane]dichloroplatinum(II) with pyridines in methanol. Effect of basicity and steric hindrance

Abstract

The kinetics of the forward and reverse steps of the process [Pt(PhSCH₂CH₂SPh)Cl₂]+ am ⇌[Pt(PhSCH₂CH₂SPh)(am)Cl]⁺+ Cl^–(am = one of a number of pyridines and other heterocyclic nitrogen bases 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 nature of the entering pyridine, and steric hindrance due to the presence of one or two methyl groups in α position to the nitrogen markedly decreases the reactivity. The first- and second-order rate constants for the reverse reaction are very sensitive to the basicity of the leaving group and a plot of log k₂^r against the pK_a, of the conjugate acids of unhindered pyridines is linear with a slope of –0.56. Steric retardation for monosubstituted α-methylpyridines is relatively small. The equilibrium constants for these reactions have been determined from the ratio of the rate constants and a plot of log K against the pK_a. of the unhindered pyridines is linear with a slope of 0.58. The results are compared with data from the literature and discussed in terms of the reaction profile.