a critical review of the phenomenon of bond-stretch and distortional isomerism in oxo and sulphido complexes of transition metals

This thesis describes studies directed towards the preparation of oxo compounds of the early transition metals with a view to assessing their potentials for exhibiting distortional and bond-stretch isomerism. Analogous sulphido complexes in the case of the M(E)X(_3)(PMe(_3))(_3) system (M= Nb, Ta; X= CI; E= S) have also been investigated in detail to provide a contrast with another chalcogenide element and further aid an understanding of the phenomenon of bond-stretch isomerism. Chapter 1 reviews the body of evidence to date on the phenomena of distortional and bond-stretch isomerism, and considers possible theoretical origins for the phenomena. Chapter 2 describes the detailed study of distortional isomerism in the six coordinate series of complexes Mo(O)X(_2)(PMe(_2)Ph)(_3) (X= CI, Br, I) and confirms the existence of two pure blue forms for the chloro and bromo cases. Chapter 3 assesses the evidence for further examples of distortional isomerism in the complexes W(O)Cl(_2)(PMe(_2)Ph)(_3), Mo(O)(NCS)(_2)(PMe(_2)Ph)(_3) and Mo(O)Cl(_2)(Pet(_2)Ph)(_3). Structural characterisation by X-ray crystallography has indicated the existence of a link between the orientation of the aikyl and aryl substituents on the phosphine ligands and the general features observed in their infrared spectra. Chapters 4 and 5 describe the seven coordinate oxo and sulphido-halide derivatives of niobium and tantalum which exhibit the phenomenon of bond-stretch isomerism. Compounds studied include M(E)X(_3)(PMe(_3))(_3) (M= Nb, E= O, S, X=C1, Br; M= Ta, E= S, X=Cl). The structures of these compounds have been re-investigated by X-ray crystallography in order to assess the significance of the co-crystallised isostructural NbCl(_4)(PMe(_3))(_3) contaminant. Chapter 6 summaries the evidence for and against distortional and bond-stretch isomerism. Chapter 7 gives experimental details for chapters 2-5.