- Kinetics and mechanism for reduction of trans-dichloro-tetracyanoplatinate(IV) by tetraammineplatinum(II) and bis (ethylenediamine) platinum(II)
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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 kobsd=(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...PtII...Cl...Pt IV...Cl] ≠ for the k1 pathway, or [Cl...PtII...Cl...PtIv...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 dz2 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-PtIv 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.
- Shi, Tiesheng,Elding, Lars I.
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- KINETIC STUDIES ON THE OXIDATION OF ETHYLENEDIAMINEBIS(PYRIDINE)-AND BIS(ETHYLENEDIAMINE)-PLATINUM(II) COMPLEXES BY HEXACHLOROIRIDATE(IV)
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The oxidation reactions of (2+) by IrCl6(2-) in the presence of chloride ions have been kinetically investigated in aqueous solution.In the absence of added IrCl6(3-) the reactions obey the rate law: -d/dt = 2(ka + kb)a further rate term, second order in A consistent reaction mechanism is proposed.The complex (2+) reacts much faster than (2+), e.g. at 20 deg C kb/dm6mol-2s-1 = 2.81 x E4 (L = 1/2 en) or 5.7 x E1 (L = py), owing to a very favourable activation enthalpy -1 = 2.0 (L = 1/2 en) or 26.2 (L = py)>.The kinetic and thermodynamic tendencies towards the oxidation of such complexes appear to be correlated.
- Peloso, Arnaldo
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p. 1473 - 1476
(2007/10/02)
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