Electrochemical and Theorectical Studies of Oxidation-Reduction Reactions of Platinum (II) and (IV) Complexes
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2014-01-15
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University of Hawaii at Manoa
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Electrochemical interconversion of complexes of Pt(II) and Pt(IV) is accelerated by the presence of halideions and halogen ligands. Semi-empirical MO Calculation and spectra data suggest that oxidation of PT(II) involves the transfer of electron density from filled orbitals of the complex, such as a1g( σ*) [5dz2] through unfilled orbitals of the compact layer or the electrode surface. Similarly, reduction of Pt(IV) complexes, for instance those of D4h symmetry such as PtIVL4X4, may proceed through interaction of a low-energy unfilled orbital if Pt(IV), such as a1g( σ*), with filled orbitals of the compact layer or the electrode surface by way of the unfilled ndz2 orbital of a bridging halide ligand. Pt(II) complexes containing unsaturated ligands such as CN of SCN- are less rapidly oxidized than their saturated analogs. Evidence is presented that unsaturated ligands having suitable π* orbitals stabilize the particular orbitals from which electron density is removed in the rate-limiting step. Pt(IV) complexes having a CN- ligand situated trans to a lone halide ligand are reduced with difficulty, whereas complexes having a trans-pair of halide ligands react readily; this trend is consistent with stabilization of the leaving trans-axis and de-stabilization of the interaction between the bridging ligand and the electrode surface due to d-π* back-bonding between Pt( IV) and the unsaturated ligand. A Molecular Orbital approach to the study of electrochemical reactions is presented. The calculated electrochemical rate constants are compared to experimental values obtained at thin-layer electrode. The qualitative trends agree in general. Suggestion for improvement of the calculation also included.
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vii, 41 pages
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