In the last decade substantial progress has been made in our understanding of uranium organometallic chemistry, specifically regarding the activation of small molecules by uranium(III) complexes. Research by Cloke and colleagues has employed dianionic eight- and monoanionic five-membered aromatic ligands around a uranium(III) centre to make mixed-sandwich systems, which have demonstrated novel reactivity with carbon monoxide and carbon dioxide. This thesis continues this work and furthers our understanding of the properties and reactivity of these complexes. The first part of this thesis describes the preparation of novel mixed-sandwich complexes incorporating the COT(SiiPr3)2 ligand and either a substituted cyclopentadienyl ring or a heterocyclic analogue. The steric parameters of these complexes have been quantitatively evaluated and provide insight into the molecular structures and reactivity observed. The electronic properties of the complexes have been investigated using cyclic voltammetry and complement results obtained by other researchers from the group. The second part of this thesis describes the reactivity of the novel mixed-sandwich complexes with small molecules. Whilst it was found the cyclopentadienyl complexes exhibit similar reactivity with carbon monoxide and carbon dioxide to previously reported complexes, the heterocyclic analogues provided a wealth of new reactivity, including facile cleavage of carbon monoxide at ambient temperatures, synthesis of an unusual phosphacarbonate fragment, and unprecedented synthesis of a 2,2'-bis(arsenine) species. The third part of this thesis explores the reactivity of potential precursors to uranium(II) metallocene complexes. Uranium complexes in this oxidation state have only recently been synthesised and the formation of a uranium(II) sandwich complex is desirable for its anticipated reactivity with small molecules. Reactivity studies include the attempted reduction of uranium(III) iodide precursors with potassium-based reducing agents and the synthesis of a novel uranium(II) tetraphenylborate complex.