Kinetic and equilibrium study on the axial-ligand substitution reaction of the head-to-tail α-pyridonate-bridged cis-diammineplatinum(III) dinuclear complex: Trans effect of the axial ligand through the Pt-Pt bond to the opposite axial ligand

Nami Saeki, Yuji Hirano, Yasunari Sasamoto, Ichiro Sato, Tsuyoshi Toshida, Sousei Ito, Noriko Nakamura, Koji Ishihara, Kazuko Matsumoto

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    Abstract

    Acid dissociation constant of the axial aqua ligand in the Head-to-Tail (HT) α-pyridonate-bridged cis-diammineplatinum(III) dimer complex {[(H2O)Pt(NH3)2(μ-C5H 4NO)2Pt-(NH3)2(H2O)] 4+} was determined spectrophotometrically to be -log(Kh1/M) = 1.98±0.01 at 25°C and I = 2.00 M. Successive substitution reaction of the HT dimer with halide ions X- (X- = Cl- and Br) at the two axial sites to give monohalo and dihalo complexes was studied kinetically: [(H2O)Pt(NH3)2(μ-C5H 4NO)2Pt(NH3)2(H2O) 4+ + X- ⇄ [(H2O)Pt(NH3)2(μ-C5H 4NO)2Pt(NH3)2(X)3+ + H2O (kobs1) [(H2O)Pt(NH3)2(μ-C5H 4NO)2Pt(NH3)2(X)3+ + X- ⇄ [(X)Pt(NH3)2(μ-C5H4NO) 2Pt(NH3)2(X)]2+ + H2O (kobs2) Formation constants of the monohalo and dihalo complexes were determined spectrophotometrically to be log(K1 Cl/M-1) = 5.27±0.02 and log(K2 Cl/M-1) = 3.83±0.01 for the reaction with Cl-, and log(K1 Br/M-1) = 5.33±0.03 and log(K2 Br/M-1) = 4.44±0.02 for the reaction with Br-. In the HT dimer having two equivalent platinum atoms, the deprotonation occurs to the water molecule on one of the two Pt atoms, whereas the first nucleophilic substitution with X- occurs to the other Pt atom. Substitution of the second water ligand with Cl- proceeds by a simple substitution path, whereas the second substitution with Br- proceeds by two parallel paths: one includes the dissociation of the axially coordinated water molecule followed by the Br- coordination, and the other is the simple one-step substitution path. The difference of the reaction paths is reasonably explained by the different trans effect of the halide ions exerted through the Pt-Pt bond to the other terminal Pt atom in the monohalo complexes.

    Original languageEnglish
    Pages (from-to)2081-2088
    Number of pages8
    JournalEuropean Journal of Inorganic Chemistry
    Issue number8
    Publication statusPublished - 2001

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    Keywords

    • α-Pyridone
    • Kinetics
    • Platinum
    • Pt dimer
    • Reaction mechanisms

    ASJC Scopus subject areas

    • Inorganic Chemistry

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