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

摘要

Reduction of trans-[Pt(CN)(4)Cl-2](2-) by [Pt(NH3)(4)](2+) and [Pt(en)(2)](2+) was studied at 25 degrees C in the range 0 less than or equal to [Cl-] less than or equal to 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)=(k(1)+k(2)K[Cl-])[Pt(II)], with k(1)=400+/-10 and (3.3+/-0.4)x10(3)M(-1)s(-1), and k(2)K= (3.25 +/-0.04) X 10(4) and (6.67 +/-0.07) x 10(5) M-2 s(-1) at 25 degrees 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 k(1), together with the rapid-scan spectra, implies that reduction of trans-[Pt(CN)(4)Cl-2](2-) by these Pt(II) complexes takes place directly via chloride-bridged transition states of the type [H2O ... Pt-II... Cl ... Pt-IV... Cl](not equal) for the k(1) pathway, or [Cl ... Pt-II... Cl ... Pt-IV... Cl](not equal) for the k(2) 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(z)2 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)(4)Cl-2](2-) compared with Pt(IV) ammine complexes previously studied is rationalized in terms of transition state stabilization due to the strong sigma-donor and pi-acceptor properties of cyanide. (C) 1998 Elsevier Science S.A. All rights reserved. [References: 35]