There are numerous reports of attempts to deposit thick coatings of chromium-nickel and chromium-nickel-iron and although high quality deposits have been obtained these have been limited to very thin coatings only. The purpose of the present work was to identify and understand the controlling factors preventing continuous deposition and the build-up of thick coatings, and to develop a suitable process for such thick coatings. An amide electrolyte system was chosen asworking model for its relative simplicity and the high quality chromium-nickel deposits obtained from this system. This bath was investigated initially using conventional operating techniques (Constant Current) to establish its general characteristics. High quality deposits were obtained for thin coatings only. The need for a major change of operating philosophy was identified as essential. A number of published hypotheses explaining the failure of continuous deposition were experimentally tested and shown not to apply. Electrolytically generated chromium (II) has been identified to be an important controlling factor on the quality and composition of chromium-nickel alloys codeposited from amide electrolytes. The control of the low transient chromium (II) level, necessary for the maintenance of high quality codeposition with constant composition, can be achieved as follows. A controlled potential is required, in combination with circulation and agitation of the electrolyte and the generation of the chlorine gas as the preferred anodic product. The combination of the three techniques is necessary for the control of chromium (II) and if any of the three techniques are not employed sustained deposition fails. Chromium (II) has further been identified to catalyse an electrochemical ageing of the bath. Reproducible high quality true metallic deposits of constant composition of chromium-nickel and chromium-nickel-iron can be obtained to a thickness of several hundred microns. These have shown to display a laminar structure on etching and possible causes have been considered. Mossbauer spectroscopy and x-ray diffraction have shown that chromium-nickel-iron deposits contain a significant quantity of micro-crystalline austenite phase ~, fcc) with a dominant ferrite (e<, bcc) phase.

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Drawing inspiration from nature where enzymes containing multi-metallic active sites are ubiquitous, chemists have designed various ligands to bind more than one metal in precise structural arrangements. In Chapter One, a class of binucleating Schiff base pyrrole (Pacman) macrocycles which are both straightforward to synthesise and can be varied systematically to alter the metal environment and intermetallic separation are introduced, along with the state-of-the-art in this area. Previously reported complexes of these ligands with late transition metals, lanthanides and actinides are also reviewed. The results and discussion chapters of the thesis focus on the isolation and investigation of previously unexplored early transition-metal Pacman complexes and present new advances in low oxidation state uranium Pacman chemistry. In Chapter Two, binuclear chromium(II) complexes of two Schiff base macrocycles, H4LMe and H4LA are described. [Cr2(LMe)] features an ortho-phenyl spacer between the macrocycle donor compartments whereas the Cr(II) ions are separated by a larger anthracenyl spacer in [Cr2(LA)]. Both compounds have been characterised in solution and the solid state. Reactivity studies were carried out for [Cr2(LMe)]. Reactions of [Cr2(LMe)] with isocyanides and triphenylphosphine oxide were investigated leading to the isolation of the contrasting co-ordination compounds [Cr2(OPPh3)2(LMe)] and [Cr2(μ-CNR)(LMe)] (R = xylyl, tBu). Oxidation of [Cr2(LMe)] with I2 yields the Cr(III)/Cr(III) Pacman products [Cr2(μ- I)(I)(THF)(LMe)] and [Cr2(μ-I)(py)2(LMe)][I] when carried out in THF or pyridine, respectively. Cr(III) alkyl compounds are obtained by reaction of [Cr2(μ-I)(I)(THF)(LMe)] with the non-reducing alkyl transfer reagents MgBrEt and ZnEt2. When ZnEt2 in toluene is employed, one zinc cation is incorporated in the molecular cleft, whereas use of MgBrEt in THF yields the simple chromium alkyl complex [{Cr(Et)}2(endo-THF)(LMe)]. One ethyl group may be abstracted from [{Cr(Et)}2(endo-THF)(LMe)] by [CPh3][B(C6F5)4] to form a cationic alkyl complex. The activity of both the neutral and cationic alkyl species towards ethylene was investigated. Conclusions are discussed at the end of the chapter. Previously, investigation of low oxidation state uranium Pacman chemistry has been confined to the smaller macrocycle, H4LMe, and frequently resulted in the formation of insoluble polymeric materials that were intractable and challenging to analyse. In Chapter Three, metallation of the larger macrocycle, H4LA, with UI3 to generate a single soluble species is described, although this product could not be isolated or characterised in the solid state. A new synthesis of [U(BH4)3(THF)2] from UI3 and NaBH4 affords an alternative U(III) precursor to UI3. Metallation of H4LA using a sodium base and U(BH4)3(THF)2 yields the ionic product [Na(THF)4][{U(BH4)}2(μ-BH4)(THF)2(LA)] which was characterised in solution and the solid state. Reaction of this compound with KO(C6H2(tBu)3) forms the ligand substitution product [{U(OAr)}2(endo-BH4K)(THF)2(LA)] which undergoes selective reaction with excess S8 or CS2 to form [{U(OAr)}2(μ-S2)(LA)] and [{U(OAr)}2(μ-S)(LA)] respectively. It was discovered that the [U(BH4)3(THF)2] metallation strategy could be successfully extended to H4LMe to form [Li(THF)4][{U(BH4)}2(μ-BH4)(LMe)]. Protonolysis of the borohydride ligands of the complexes of the two different macrocycles was investigated using the weak acid [HNEt3][BPh4]. NMR spectroscopy indicated that both exo BH4 − groups in both complexes can be successively removed to generate neutral and cationic complexes but these were not isolated. Metallation of H4LA with UCl4 forms the ionic product [Li(THF)4][{U(Cl)}2(μ-Cl)3(LA)]. Various ligand substitution reactions were attempted but the only structurally characterised product was [{U(OtBu)(Cl)}{U(OtBu)(py)}(μ-Cl)(LA)], formed by reaction with KOtBu. Conclusions are discussed at the end of the chapter. Experimental and characterising data are provided in Chapter Four.

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