Oxidation of Metals

This thesis is concerned with the study the behaviour of UF6, MoF6, WF6 and NO+ towards the metals Co, Ni and Zn in CH3CN medium, and thus give a better insight into the reactivity of these hexafluorides towards first row transition elements. Solvated Co(II) has been prepared by the oxidation reactions of the metal by MoF6, WF6 and NO+. The solvated salt [Co(NCMe)6]-[PF6]2 is stable in absence of moisture and oxygen and can be stored in an argon atmosphere glove box. [Co(NCMe)6][WF6]2 and [Co(NCMe)6]-[MoF6]2 are very prone to hydrolysis and can be kept in liquid nitrogen for only one or two days. The spectra of the salts are consistent with octahedrally coordinated Co(II). Values of the ligand field splitting parameters (A) and electronic repulsion parameter (B') have been calculated and compared with those of some relevant CoL62+. They were found to fit the theoretical expectations and showed that acetonitrile is a good coordinating agent though not the strongest in the spectrochemical series. Co(II) can be electrochemically oxidised to Co(III) in cases where the counter anion is PF6-. This is not observed when WF6 - is the counter anion, which is probably due to the oxidation potentials of the Co2+/Co3+ and WF7- /W V couples which occur in the same range, hence the wave assigned to Co2+/Co3+ could not be observed. MoF6, oxidises cobalt metal to Co(II). Further oxidation, as would be predicted from the electrochemistry of [Co(NCMe) [PF6]2 was not observed, probably kinetic reasons. UF6 did not react with cobalt metal. Polymerisation of acetonitrile and reduction of UF6 took place instead. Zinc is not a transition metal but its behaviour towards the hexafluorides of uranium, molybdenum and tungsten, is similar to that of most of the first row transition elements. The metal is oxidized by MoF6, NO+ and WF6 and the salt [Zn(NCMe)6] [MF6]2 (M=Mo,P,W) is generated in solution. Coordination of acetonitrile and presence of the anion, MF6- (M=Mo,P,W), has been established from the study of its infrared and Raman spectra. UF6 does not oxidize zinc metal. Nickel metal is not oxidized by MoF6 and UF6 but it is relatively easily oxidized by NO+ and WF6 at room temperature. The solid [Ni(NCMe)6] [PF6]2 is stable at room temperature but [Ni(NCMe)6] [WF6]2 hydrolyses easily. A purple [Ni(NCMe)6] [WF6] 2 is formed when the oxidation of the metal is performed in acetonitrile solution. A fluoride ion transfer occurs in parallel, in which the hexafluorotungstate(V) anion behaves as a fluoride ion donor in acetonitrile. It reacts with WF6 to give WF7-, anion as a product. When the reaction is performed in the gas phase [Ni(NCMe)5]-[WF6]2, a white solid is formed. Neither MoF6, nor UF6, is able to oxidize massive nickel, MoF6, can,however oxidize chemically cleaned nickel metal or vacuum evaporated nickel film. A purple salt is formed; it is identified as [Ni(NCMe)6] [MoF6]2. If the nickel film is exposed to air before reaction, MoF6, does not oxidize it. This clearly shows that it is the oxide film that inhibits the reaction. On the other hand WF6, which reacts easily with massive nickel, is inert towards a nickel film free from its oxide layer. The reaction of an evaporated nickel film with WF5-18F shows that tungsten hexafluoride is adsorbed on the surface of the metal but this reaction could not be observed for kinetic reasons. Although previous workers did not take into account the role of the oxide film during the oxidation of metals, the series of reactions described above shows clearly its importance in the oxidation process. On this basis it seemed reasonable to assume that an electron transfer takes place between the oxide film and the oxidizing agent. Nickel oxide being stable, the electron transfer does not occur with MoF6. WF6 being a strong Lewis acid, it is able to form a Lewis Acid-Lewis-Base adduct which catalyses the oxidation reaction of nickel metal. Addition of pyridine to a solution mixture of nickel and MoF6 in MeCN produced a yellow solid. Its microanalysis revealed that it is in fact MoF5.2py adduct which is formed. The reaction was also performed in absence of nickel, yielding the same product, identified by microanalysis, atomic absorption and vibrational spectroscopy. Uranium hexafluoride is reported to be the strongest oxidizing agent of the series considered in this study. There was, however, no evidence of any reaction occurring between the metals, cobalt, nickel and zinc and UF6. In all cases, reduction of the hexafluoride occurred before reaction with the metal could be detected. This was followed by polymerisation of the solvent. UF6 was also reacted with a vacuum evaporated nickel film. In this case also, no reaction was observed