Electrochemical Studies of Transition Metal Hexahalometallates

[摘要] This thesis determines the redox potentials of second and third row transition metal hexabromometallates using cyclic voltammetry methods in methylene chloride solution. The results are collated with the redox potentials of second and third row transition metal hexafluoro- and hexachlorometallates and the systematic trends exhibited by all the redox potentials are discussed. The contents of this thesis are divided as follows: In chapter one the nature of transition metal hexahalometallate bonding is discussed by drawing on ideas from ligand field theory and molecular orbital theory, both of which are described briefly. The contributions to the metal-halogen bond are described separately under a) purely electrostatic effects- which are treated using electronegativity differences, and b) covalency effects which are subdivided into sigma-covalency and pi-bonding. Lastly, chapter one includes a brief description of the vibrational and electronic spectra of transition metal hexahalometallates as these two methods were the principle means of identifying complexes. Chapter two begins with a conceptual description of charge transfer reactions at electrode surfaces and then approaches the same subject mathematically to derive the Butler-Volmer equation, the fundamental equation of electrode kinetics. The D. C. and A. C. cyclic voltammetry techniques are described and for a D. C. cyclic voltammogram, the current-potential profile is discussed and illustrated for reversible, quasi-reversible and irreversible electron transfer processes. The effects of coupled chemical reactions and adsorption of reactant and/or product on the electrode surface are also reviewed. Chapters three and four respectively describe the cyclic voltammetric investigations of the hexabromometallate complexes prepared in this work and the halogen/halide systems of chloride, bromide and iodide. The electrode potentials determined are compared with literature values wherever possible. Complex electrochemical activity at high oxidation potentials thwarted the detection of any of the possible [MBr6]0/1- redox couples. Adsorption of hexabroinometallate species onto the electrode at high oxidation potentials is put forward as a likely reason. Other problems which were encountered include the reaction of tungsten hexabromide with the supporting electrolyte and the propensity of trivalent hexabromometallate anions to dimerize when countered by the sterically large tetra-n-butylammonium cation. In chapter five, the electrode potentials of the transition metal hexabromometallate complexes determined in chapter three are collated with the electrode potentials of hexafluoro- and hexachlorometallate complexes already determined by previous workers. The systematic trends which emerged include 1. The electrode potentials of isovalent [MX6]z/z-1 redox couples increase regularly across a period except for a discontinuity at the d3/4 couple. 2. The discontinuity is more pronounced for second row than third row transition metal hexahalometallates. 3. [MX6]z complexes become more oxidizing with increasing oxidation state of the metal. 4. Third row transition metal hexahalometallate complexes are more reducing than their isoelectronic second row counterparts. By using simple thermodynamic arguments it is derived that the standard electrode potential of a redox couple is proportional to the free energy difference between the reduced and the oxidized species, which is in turn composed of two separate contributions: The entropy- which was associated with alterations to the solvation sphere which, it was concluded, have no influence on the systematic trends observed. The enthalpy- of which the principal contributions arise from 1. the ionization enthalpy of the metal. 2. the dn configuration of the metal. 3. the nature of the halide ligand. 4. the nature of the solvent. Chapter five concludes with a brief survey of some of the uses and applications of hexahalometallate redox potentials. Finally, chapter six details the synthesis and characterisation of the compounds studied in this thesis. Included is a description of the electrochemical cell and its manipulation, and the preparation of the solvent and supporting electrolyte.

[发布机构] University:University of Glasgow

[关键词] Inorganic chemistry