| الوصف: |
This thesis concerns the reduction induced substitution reactions of three families of transition metal salts: (i) [MX6]2-, where M = Ir, Os, Re and X = Cl-, Cr-, l-, (ii) OsX3Y3], where X = Cl- or Br- and Y = a tertiary alkyl phosphine or arsine, and (iii) techniques, in particular double-step chronoamperometry, to elucidate the mechanisms involved and the factors influencing the rate of reaction and the activation energy for the process. On one-electron reduction, [MX6]2- undergoes a substitution reaction of one halide, X, for a ligand, L, of less electron donating character. The rate constant and activation energy are largely independent of the nature or the concentration of the entering group, L, and inhibited by an increased concentration of the leaving group in solution. The rate limiting step is the loss of halide. The rate increases Cl- < Br- < I-, reflecting leaving group ability and ease of solvation in the organic solvent. On changing the metal centre, the rate of reaction decreases Re>>Os>Ir, rationalised in terms of the nuclear charge of the metal centre: Re(III) is less able to support the electron density from the halides than Os(III) and Ir(III). The activation energy is understood to be the energy required to break the metal-halide bond and represents the extent of orbital overlap between the metal and halide. The nephelauxetic parameter, b, indicates strongly covalent character of the iridium-halide bond reflecting a high overlap of the iridium and halide orbitals. Overall, a Dissociative mechanism has been assigned to the halide loss with the exception of [OsCl6]2- which can be more adequately described by a dissociative Interchange mechanism with the formation of a pre-equilibrium complex, {[OsCl6]3-,L}. Digital simulation of the mechanisms supports these findings. |
