This thesis is concerned with the synthesis and characterisation of new ansa bridged bis-η-cyclopentadienyl derivatives of niobium. Chapter 1 provides an introduction to the area of early transition metal bent metallocene derivatives and charts the development of ansa bridged metallocenes. Chapter 2 presents the synthesis and characterisation of some new ansa niobocene derivatives prepared from niobium(IV) chloride. The compounds [Nb(η-C₅H₄-CMe₂- η-C₅H₄)(η²-BH₄)],* [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(PMe3)H],* [Nb(η-C₅H₄-CMe₂-η- C₅H₄)(PPh₃)H], [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(SPh)₂] and [Nb(η-C₅H₄-CMe₂-η- C₅H₄)(CH2Ph)2]* are described. An asterisk indicates that the X-ray crystal structure was determined. Chapter 3 describes the synthesis and characterisation of some ansa niobocene derivatives containing ancillary imido ligands. The compounds [Nb(η-C₅H₄-CMe₂-η- C₅H₄)(NSiMe₃)Cl],* [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(NBu')X] (X = Cl,* Br,* I,* Me), [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(NHBu')Cl][BF₄)]and[Nb(η-C₅H₄-CMe₂-η-C₅H₄)(O)Cl]* are described. An asterisk indicates that the X-ray crystal structure was determined. Photoelectron spectra were recorded for [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(NBu')X] (X = Cl, Me) and compared to a geometry optimisation calculation obtained using density functional theory. In these formally twenty electron compounds the HOMO has negligible metal contribution and the eighteen electron rule is not violated. Chapter 4 describes the results of a preliminary investigation into ansa niobium(III) bis-η-cyclopentadienyl derivatives containing phosphorus donor ligands. The compound [Nb(η-C₅H₄-CMe₂-η-C₅H₄)(PMe₃)Cl] is characterised spectroscopically. The synthesis and full characterisation details of the compound [Nb(η-C₅H₄-CMe₂-η- C₅H₄){P(OMe)₃}Cl] are given, including the X-ray crystal structure. Chapter 5 provides the experimental details for the preceding Chapters and Chapter 6 contains the characterising data for the new compounds described in this thesis. Full details of the X-ray crystallographic structure determinations are given in Appendices A -1.