This thesis describes the synthesis, characterisation and reactivity of a series η5-coordinated thiophene complexes of ruthenium and osmium metal centres. Chapter 1 reviews the background to this chemistry and includes a discussion on the motivation behind this research. The coordination chemistry relating to mononuclear transition metal complexes of thiophenes are reviewed, along with an extensive examination of the synthesis and reactivity of mononuclear ruthenium and osmium complexes containing thiophenes. Chapter 2 describes an examination of the coordination chemistry of the [{Ru(η5-C4Me4S)Cl(μ-Cl)}2] dimer. The synthesis and characterisation of a series of novel complexes of η5-tetramethylthiophene-ruthenium(II) containing mono-, bi- and tridentate coligands are described. The compounds [Ru(η5- C4Me4S)Cl2(PPhMe2)], [Ru(η5-C4Me4S)(bipy)(PPhMe2)][PF6]2, [RU(η5- C4Me4S){K3-HB(Pz)3}][PF6] and [Ru2(η5-C4Me4S)2(μ-Cl)3][PF6] were characterised crystallographically. Chapter 3 reports an investigation into the nucleophilic reactivity of a series of tetramethylthiophene complexes, [Ru(η5-C4Me4S)(L3)]2+ (L3 = HC(Pz)3, [9]aneS3, [2.2]-paracyclophane). In general these complexes undergo nucleophilic addition reactions, in which exo-attack occurs at the carbon of the '2' position forming an η4-thiopheneyl complex of the type [Ru(η4-C4Me4S-2- Nuc)(L3)]+. The crystal structures of [Ru(η4-C4Me4S-2-OEt)([9]aneS3)][PF6] and [Ru(η4-C4Me4S-2-H)([9]aneS3)][PF6] are presented. Chapter 4 describes the synthesis and reactivity of a series of mixed sandwich compounds of the type [Os(η5-Th)(η6-p-cymene)][CF3S03]2 (Th = C4H4S, 2,5- Me2C4H2S, C4Me4S). These compounds were found not to react cleanly with nucleophiles, but did react via base hydrolysis to give acyl-thiolate complexes of the type [Os(σ,η3-SC3R3C(O)R)(η6-p-cymene)]. The compound [Os(η5- C4Me4S)(η6-p-cymene)][CF3S03]2 was characterised crystallographically.