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Title: Spectroscopic studies to establish relationships between structure, bonding and reactivity in selected inorganic species
Author: Pritchard, Helen Margaret Inskip
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 2005
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This thesis explores the influence of molecular and electronic structure on the reactivity of some small inorganic complexes, beginning with a review of the nature and properties of metal-hydrogen and metal-carbon bonds of early transition metals and of their importance in catalysis. A series of cationic niobium methyl complexes, TpMe2NbMe(L)(MeC=CMe) BArf4 (L = OEt2, THF, 0'Pr2, PMe2Ph and PEt/j) was synthesised and characterised by NMR spectroscopy. Small-scale reactions at low temperatures resulted in the formation of a highly unstable cation, which was stabilised by co-ordination with a series of small electron donor ligands (L). Low temperature NMR studies were used to characterise the resulting air- and temperature-sensitive complexes in solution. One adduct (L = OEt2) was isolated in the solid state and its structure, determined by X-ray crystallography, accorded with the interpretation of its NMR spectrum. The influence of L on the reactivity and stability of the complexes was explored, with preferred co-ordination demonstrated through ligand exchange reactions. Initial catalytic assays established a level of activity warranting further investigation. A high quality crystal of (Bz3Ti)20 was sujected to an experimental charge density study, assisted by an 'Atoms in Molecules' (AIM) theory analysis. This yielded insights into the charge distribution at the Ti and O centres demonstrating the presence of 'ligand induced charge concentration' (LICC). Density functional theory (DFT) calculations were carried out on (Bz3Ti)20 producing theoretical charge densities in excellent agreement with the experimental results. X-ray crystallography studies were conducted on the intermediates in the synthesis of MeTi(NMe2)3, BrTi(NMe2)3 and the novel complex {Ti(NMe2)20}2Ti3(NMe2)60Br2. IR and NMR spectroscopies were used to explore the nature of the Ti-Me moiety in MeTi(0'Pr)3 no a-agostic interactions were discovered. Variable energy photoelectron spectroscopy has been used for an in-depth study of the electronic structure, including core electrons, of some small, tetrahedral inorganic compounds. The bonding in the industrially used catalyst MeRe03 was studied through the variation of the valence electron band intensities over a wide range of photon energies. Investigation of the electronic structure at the metal centres of the series of metalloborohydrates Zr(BH4)4, Hf(BH4)4 and U(BH4)4 was carried out in a similar manner. Calculation of relative partial photoionisation cross-sections and derived branching ratios along with observation of features such as shape and giant resonance gave insight into the relative ligand and metal contributions to the orbitals. The core electrons and absorption edges of the complexes were also studied. Experimental results showed good correlation with density functional theory calculations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available