The transmission of mesomeric effects through palladium and platinum: pyridine proton shifts in complexes of the type trans-[PtCl(py)L]

Abstract

In an attempt to distinguish inductive from mesomeric effects, pyridine proton chemical shifts in complexes of the type trans-[PtCl₂(py)L] have been measured where the ligands L are alkyl-phosphines,-arsines,-stibines, sulphides, selenides and tellurides, and ethylene, also in the halide series trans-[PtX₂py(PPrⁿ₃)₂](X = Cl, Br, or l). These differ from pyridinium salts in showing smaller downfield shifts for the β- and γ-protons than for the α-protons, a possible diagnostic test for ‘class b’ metal ions. In the trans-[PtCl₂(py)L] series of complexes the β-proton shifts are roughly constant, indicating that inductive and mesomeric effects are probably balanced and small at this position. A comparison of the shifts in the halide series reveals a trend to lower field with increasing electronegativity of X. However, all differences in the shifts are small and appear to have no quantitative significance as a measure of mesomeric effects, although the positive value of the differences between the downfield proton shifts at the γ position relative to the β suggests that mesomeric electron withdrawal does take place.