Add time:08/08/2019         Source:sciencedirect.com

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 kobnd = (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…PtIV…Cl] for the k1 pathway, or [Cl…PtII…Cl…PtIV… reaction rate of [Pt(en)2]2− compared with [Pt(NH3)4]2− is due to a higher electron density in the d2 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 oof [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.

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