Reactivity of analogues of Zeise’s anion with different ligands trans to the h2-ethene

The Zeise’s salt, K[PtCl3(h2-C2H4)], is a paradigmatic example of the olefin to metal bond and still an important starting material for the synthesis of platinum-based organometallic compounds. The great reactivity of Zeise’s anion, essentially related to the lability of the chlorido ligand trans to the olefin, which can be easily substituted by any ligand having a reasonable affinity for platinum. To get further insights in the reaction of nucleophilic substitution upon changing the ligand trans to the h2-olefin, we tested the reactivity of three monoanionic platinum(II) substrates (Zeise’s anion itself, [PtCl3(h2-C2H4)]-, 1, trans-[PtCl2(OH)(h2-C2H4)], 2, and trans-[PtCl2(h1-CH2NO2)(h2-C2H4)]-, 3), towards aromatic imines with different steric requirements (pyridine, 4-methylpyridine, and 2,6-dimethylpyridine). We also performed a X-ray crystal structure characterization of the tetraphenylphosphonium salts of 2 and 3. Our data have highlighted the nature of the Pt-Cl Pt-OH, and Pt-C s bonds. Cl– is the only one of the three trans-to-olefin ligands which can have a h-acceptor capacity towards the metal. The s-donor capacity of the ligands can be ranked in the order: CH2NO2 C- >> Cl– > OH–. The carbanion has an unexpectedly high weakening effect on the bond between platinum and the trans-olefin. As a consequence the olefin is displaced in preference not only by another olefin but also by h-donors such as pyridines. In the light of the present results, the series ranking the trans-effect, which were set up in the 1980’s, needs to be reconsidered in view of the importance for synthetic chemistry of trans-directing ligands.