Kinetic data for outer-sphere V2+ and [Ru(NH3)6]2+ reductions of platinum(IV) complexes and a correlation of rate constants

Abstract

The kinetics of 2 : 1 V²⁺ and [Ru(NH₃)₆]²⁺ reductions of six Pt^IV complexes: trans-[Pt(NH₃)₄Cl₂]²⁺, trans-[Pt(NH₂CH₃)₄Cl₂]²⁺, [Pt(NH₃)₅Cl]³⁺, [Pt(NH₃)₅Br]³⁺, mer-[Pt(NH₃)₃Cl₃]⁺, and [PtCl₆]^2– have been studied. Reactions involve one-equivalent changes, Pt^IV→ Pt^III→ Pt^II where the first step is rate determining and Pt^III is a transient intermediate. A linear correlation of second-order rate constants (15 °C)k_V and k_Ru, log k_V= 0.89 log k_Ru– 1.68, is obtained at I= 0.10M(LiClO₄),consistent with outer-sphere reactions, and non-utilization (by V²⁺) of inner-sphere pathways involving Cl^– and Br^– bridges. The occurrence of outer-sphere reactions appears to preclude two-equivalent changes. Rate constants for the V²⁺ reductions of trans-[Pt(NH₃)₄Cl₂]²⁺ > [Pt(NH₃)₅Cl]³⁺ differ by a factor of 60, whereas those for corresponding [Pt(NH₃)₄]²⁺ reductions, which involve two-equivalent changes, have previously been shown to differ by a factor 10⁴. Activation parameters have been determined for five of the V²⁺ reductions. Those for the reduction of trans-[Pt(NH₃)₄Cl₂]²⁺ and trans-[Pt(NH₂CH₃)₄Cl₂]²⁺ differ significantly indicating an influence of the methyl groups.