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The work described in this Thesis is a study of the synthesis and reactivity of some giant osmium carbonyl clusters and an investigation into their magnetic and electronic properties. Chapter One contains an overview of the concept of the chemical bond from Dalton to the present day ideas of Wade and Mingos, and their application to transition meial clusters. Chapter Two is an investigation into the fluxional and dynamic behaviour of mixed-metal clusters in solution using multinuclear N.M.R. (lH,iaC, and 3lP{1H}). The clusters [Os10C(CO)24MPR3]-(M = Cu, Ag or Au) and [Os1oC(CO)24(MPR3)(M>PR3)] (M = M'= Ag or Au, and M 5* M' Ag and Au) have been examined using these techniques. Chapter Three reports the synthesis of the dianion [Os11C(CO)27]2~, a cluster containing a trigonal prismatic geometry and its reaction with elec-trophiles such as H+, I2, [Cu(NCMe)4]PF6, AuPR3Cl and AgBF4. THe X-ray structure of [HOsnC(CO)27]~ is described, and the conversion of [OSllC(CO)27]2- to [Os10C(CO)24]2-. Chapter Four is concerned with the synthesis of giant osmium clusters using [Os10C(CO)24]2"~ as a precursor and reacting it with Hg(CF3COO)2 to form [Os20Hg(C)2(CO)42]2-, [Os30Hg2(C)3(CO)72]2-and [Os40Hg3(C)4(CO)oe] • The corresponding gold derivatives, [Os10C(CO)24AuBr]- and [Os20Au(C)2(CO)40]2~ have also been prepared and fully characterised by X-ray crystallography. Chapter Five describes the use of l87Os-1H satellites and Inversion-Recovery 1H n.m.r. as aids in the study of the dynamic behaviour of hydrido osmium carbonyl clusters in solution. In particular the work has focussed on differentiating between hydride ligands in tetrahedral or octahedral interstitial sites. The improved synthesis of [H4Osio(CO)24]3~ is reported and its reactions with electrophiles are discussed. The dianion reacts with H+ to form [H5Os10(CO)24]~ which has been characterised by an X-ray structural determination. Chapter Six presents UV/visible, magnetic and EPR measurements on a range of osmium carbonyl clusters prepared in Chapters Two to Five. A molecular orbital framework for understanding these results is presented. Finally Chapter Seven contains the experimental section and spectroscopic data.

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Multipulse-excitation has greatly improved the speech quality achievable from linear predictive coders which previously required speech to be classified as voiced or unvoiced for excitation purposes. Multipulse removes the need for voicing classification, improving speech quality by enhancing the excitation and offsetting errors in the vocal tract filter. An investigation of multipulse-excitation applied to a channel vocoder and a formant synthesiser was conducted. The prime objective was to improve the performance of these algorithms and achieve multipulse linear prediction speech quality, our target quality. This dissertation outlines and restates the idea of multipulse-excitation applied to a linear predictive vocoder. We then examine a high quality channel vocoder and formant synthesiser, and the use of multipulse-excitation to improve their performances. In each case time and frequency domain multipulsecalgorithms were used. Various modifications were made to these algorithms in order to accommodate multipulse-excitation and improve the overall speech quality. In the case of the channel vocoder this involved a novel technique, which sacrificed the inherent waveform preserving properties of the multipulse algorithm. Only by increasing both the pulse rate and the number of channels could the multipulse-excited channel vocoder achieve our target quality. With the formant synthesiser it was possible, by variation of the pulse rate alone, to achieve our target quality. Comparisons are drawn between the three multipulse algorithms and reasons given for their differing performance; this is substantiated by experimental results. These results suggested interesting improvements to the multipulse-excited formant synthesiser; and also hinted at a new and novel technique for formant tracking, using multipulse-excitation applied to a formant synthesiser.

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