Kinetics and mechanism for reduction of trans-dichloro-tetracyanoplatinate(IV) by tetraammineplatinum(II) and bis (ethylenediamine) platinum(II)

Article abstract of DOI:10.1016/S0020-1693(98)00199-6

Reduction of trans-[Pt(CN)₄Cl₂]^2- by [Pt(NH₃)₄]²+ and [Pt(en)₂]²⁺ was studied at 25°C in the range 0 ≤ [Cl^-] ≤ 0.4 M by use of stopped-flow spectrophotometry. The stoichiometry [Pt(IV)]:[Pt(II)] is 1:1. Rapid-scan spectra show clear-cut isosbestic points, indicating that no reaction intermediates are accumulated to a significant amount. For pseudo first-order conditions with excess Pt(II), the observed rate constants can be expressed as k_obsd=(k₁+k₂K[Cl^-]) [Pt(II)], with k₁=400±10 and (3.3±0.4) × 10³ M^-1 s^-1, and k₂K=(3.25±0.04) × 10⁴ and (6.67±0.07) × 10⁵ M^-2 s^-1 at 25 °C for reduction by [Pt(NH₃)₄]²⁺ and [Pt(en)₂]²⁺, respectively. The mechanism involves two parallel reaction pathways. The analysis of the ionic strength dependence of k₁, together with the rapid-scan spectra, implies that reduction of trans-[Pt(CN)₄Cl₂]^2- by these Pt(II) complexes takes place directly via chloride-bridged transition states of the type [H₂O...Pt^II...Cl...Pt ^IV...Cl] ≠ for the k₁ pathway, or [Cl...Pt^II...Cl...Pt^Iv...Cl] ≠ for the k₂ path. The higher reaction rate of [Pt(en)₂]²⁺ compared with [Pt(NH₃)₄]²⁺ is due to a higher electron density in the d_z2 orbital making it more readily available to accommodate the Cl⁺ leaving from the platinum(IV). The redox rates depend strongly on the thermodynamic driving force, reflecting a significant weakening of the Cl-Pt^Iv bonds in the transition states. The much faster reduction of [Pt(CN)₄Cl₂]^2- compared with Pt(IV) ammine complexes previously studied is rationalized in terms of transition state stabilization due to the strong σ-donor and π-acceptor properties of cyanide.