Reaction Mechanism for Olefin Exchange at Chloro Ethene Complexes of Platinum(II)

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• 作者：Maria Rosaria Plutino ; Stefanus Otto ; Andreas Roodt ; and Lars I. Elding
• 刊名：Inorganic Chemistry
• 出版年：1999
• 出版时间：March 22, 1999
• 年：1999
• 卷：38
• 期：6
• 页码：1233 - 1238
• 全文大小：88K
• 年卷期：v.38,no.6(March 22, 1999)
• ISSN：1520-510X

文摘

Complex equilibria in methanol/chloroform/dichloromethane solutions containing Zeise's anion, [PtCl₃(C₂H₄)]^-(1), the solvento species, trans-[PtCl₂(C₂H₄)(MeOH)] (2), and the dinuclear complex, trans-[PtCl₂(C₂H₄)]₂ (3),have been studied by UV-vis, ¹H, and ¹⁹⁵Pt NMR spectroscopy, giving average values of K_Cl = (1.6 ± 0.2)10³M^-1 and K_S = (0.16 ± 0.02) M^-1 for the equilibrium constants between 2 and 1 and 3 and 2, respectively. Thebridged complex 3 is completely split into monomeric solvento complexes 2 in methanol and in chloroform ordichloromethane solutions with [MeOH] > 0.5 M. Ethene exchange at the mononuclear complexes 1 and 2 wasstudied by ¹H NMR line-broadening experiments in methanol-d₄. Observed overall exchange rate constants decreasewith an increase in free chloride concentration due to the displacement of the rapid equilibrium between 1 and2 toward the more slowly exchanging parent chloro complex 1. Ethene exchange rate constants at 298 K forcomplexes 1 and 2 are k_ex1 = (2.1 ± 0.1)10³ M^-1 s^-1and k_ex2 = (5.0 ± 0.2)10⁵ M^-1 s^-1, respectively, withcorresponding activation parameters mages/gifchars/Delta.gif" BORDER=0 >H₁^{mages/entities/thermod.gif">} = 19.1 ± 0.3 kJ mol^-1, mages/gifchars/Delta.gif" BORDER=0 >S₁^{mages/entities/thermod.gif">} = -117 ± 1 J K^-1 mol^-1, mages/gifchars/Delta.gif" BORDER=0 >H₂^{mages/entities/thermod.gif">} = 10.2± 0.4 kJ mol^-1, and mages/gifchars/Delta.gif" BORDER=0 >S₂^{mages/entities/thermod.gif">} = -102 ± 2 J K^-1 mol^-1. The activation process is largely entropy controlled; theenthalpy contributions only amounting to mages/entities/ap.gif">30% of the free energy of activation. Ethene exchange takes place viaassociative attack by the entering olefin at the labile site trans to the coordinated ethene, which is either occupiedby a chloride or a methanol molecule in the ground state. The intimate mechanism might involve a two-stepprocess via trans-[PtCl₂(C₂H₄)₂] in steady state or a concerted process via a pentacoordinated transition statewith two ethene molecules bound to the platinum(II).