The cis- and trans-effects of cyanide in substitution at platinum(II)

Abstract

The cis- and trans-[Pt(am)₂Cl₂] isomers (am = dimethylamine, pyridine, 4-cyanopyridine, 4-chloropyridine, 2-methylpyridine, 4-methylpyridine, 2,4-dimethylpyridine, 4-ethylpyridine, morpholine or piperidine), react rapidly with excess CN^– in methanol to form the corresponding cis- and trans-[Pt(am)₂(CN)₂] species which then react further to give [Pt(CN)₄]^2–. The kinetics of the slow step, [Pt(am)₂(CN)₂]+ CN^–→[Pt(am)(CN)₃]^–+ am, has been studied. In spite of the strong trans effect of CN^– the trans species are all more reactive than the corresponding cis isomers. The second-order rate constants are sensitive to the nature of am; plots of log k₂ against the pK_a of Ham⁺ are linear but the slope for the trans isomers (–0.27) is much greater than that for the cis isomers (–0.07). The rate constants for the displacement of o-methyl substituted pyridines are smaller than predicted from their basicity. This steric hindrance effect is much more marked in the trans isomers. The complexes with am = NH₃ and NH₂Me are similarly less reactive than might be predicted but the effect is the same in both isomers. The results are explained in terms of the trans effect of cyanide and its effect upon the intrinsic reactivities and nucleophilic discriminations of the substrates.