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Axial ligand substitution reaction kinetics of pyrimidine-2-thionato bridged binuclear platinum(iii) complexes

Göy, Aytunç
The kinetics of the ligand substitution reactions, which is represented by the equation, [Pt2(C4H3N2S)4X2] + 2Y- Pt2(C4H3N2S)4Y2 + 2X- where X- = Cl-, Br-, I- and Y- = Cl-, Br-, I- are studied in acetonitrile in the presence of excess Y- ion concentrations, under constant ionic strength. All reactions are reversible. The rate of the above reaction is dependent on binuclear complex and entering ligand concentrations. Thus general rate equation can be written as Rate = k [Y-]a[Pt2(C4H3N2S)4X2]b The reaction rates are first order with respect to the substrate complex (b=1). The experimentally determined values of the order of the reaction with repect to entering ligand, “a”, are 0.96±0.057 (X=I-, Y=Cl-), -0.49±0.037 (X=Cl-, Y=I-), 0.28±0.023 (X=I-, Y=Br-), 0.48±0.044 (X=Br-, Y=I-), 0.53±0.042 (X=Br-, Y=Cl-), and -0.21±0.014 (X=Cl-, Y=Br-). The rate constants are 12.1±2.05 M-1s-1 (X=I-, Y=Cl-), (5.7±1.6)x10-3 M1/2s-1 (X=Cl-, Y=I-), 0.3±0.27 M-0.3s-1 (X=I-, Y=Br-), 0.53±0.11 M-1/2s-1 (X=Br-, Y=I-), 1.74±0.16 M-1/2s-1 (X=Br-, Y=Cl-), and 1.71±0.37x10-2 M0.2s-1 (X=Cl-, Y=Br-). To obtain information about the energetics of the reactions, the temperature dependence of the rate constants is determined and the activation parameters ΔH* and ΔS* are calculated. The values ΔS* are negative and, in the range of -81 and -236 J K-1 mol-1. These results support an associative-interchange, Ia, mechanism. All data obtained in this work are used to propose a mechanism which will be consistent with the experimentally determined rate law.