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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.

To get further insight in the reaction of nucleophilic substitution upon changing the ligand trans to a η(2)-olefin, the reactivity of some monoanionic platinum(II) complexes (trans-[PtCl(2)X(η(2)-C(2)H(4))](-), X = Cl(-), 1, OH(-), 2, and CH(2)NO(2)(-), 3) towards pyridines with different steric hindrance (py, 4-Mepy, and 2,6-Me(2)py) has been tested. All crystallographic (2 and 3 reported for the first time) and spectroscopic data are in accord with a platinum-olefin interaction decreasing in the order 2 > 1 > 3, paralleling the decreasing electronegativity of the donor atom (O > Cl > C). Not only the platinum-olefin bond but also the bond between platinum and the ligand trans to the olefin appear to be strongest in 2 (Pt-O distance at the lower limit for this type of bond). In the reaction with py, the ligand trans to the olefin is displaced in 1 and 2. Moreover the reaction is in equilibrium in the case of sterically hindered 2,6-Me(2)py, the equilibrium being shifted moderately or prevalently toward the reagents in the case of 1 and 2, respectively. In the case of 3, the reaction with pyridines leads to substitution of the olefin instead of the carbanion. This is in accord with the observation that carbanions strongly weaken the trans Pt-olefin bond.