The principal aim of this thesis is the synthesis and characterisation of four-membered metallacyclic complexes. Reactions of square-planar palladium(II), platinum(II) and gold(III) dihalide complexes with a variety of amides, some in the presence of silver(I) oxide are discussed. Chapter one reviews the literature of small-ring metallacyclic chemistry, in particular the synthesis of four-membered metallacycles, as they are, in the main, the complexes that are further reported throughout the remainder of the thesis. Chapter two reports the synthesis of four-membered metallalactam complexes [M{N(Ph)C(O)CHC(O)Ph}L₂] (2.11 - 2.15) formed from the reactions of square-planar L₂MCl₂ complexes [M = Pt, Pd, L = PPh₃, L₂ = 1,5-cyclooctadiene (COD), 1,2-bis(diphenylphosphino)ethane (dppe), 2,2’-bipyridine (bipy)] with 2-benzoylacetanilide [PhC(O)CH₂C(O)NHPh] in the presence of silver(I) oxide. The X-ray crystal structure of [Pd{N(Ph)C(O)CHC(O)Ph}(bipy)].CH₂Cl₂ (2.14.CH₂Cl₂) was determined to confirm the geometry of the compounds. The reactions of acetoacetamide [NH₂C(O)CH₂C(O)CH₃] with cis-[PtCl₂ (PPh₃)₂], [PtCl₂(dppe)] and [PdCl₂(bipy)] (2.16 - 2.18) in the presence of silver(I) oxide are also discussed. Chapter three describes the isolation and characterisation of a platinum(II)-amidate complex, believed to be the first characterised intermediate in silver(I) oxide mediated reactions. For short reaction times the complex cis-[Pt{N(C(O)CH₂CN)(CO₂Et)}Cl(PPh₃)₂] (3.4) can be isolated from the reaction mixture of cis-[PtCl₂(PPh₃)₂] with N-cyanoacetylurethane [NCCH₂C(O)NHCO₂Et] in the presence of silver(I) oxide. For long reaction times this synthesis is known to afford the four-membered metallalactam [Pt{N(CO₂Et)C(O)CHCN}(PPh₃)₂]. Chapter four details the synthesis and characterisation of four-membered gold(III) lactam complexes. A series of metallalactam complexes were characterised from the reaction of [LAuCl₂] [L = {C₆H₃(CH₂NMe₂)-2-OMe-5} or {C₆H₄(CH₂NMe₂)-2}] with N-cyanoacetylurethane, 2-benzoylacetanilide, acetoacetanilide or acetoacetamide in the presence of silver(I) oxide. A representative of these gold(III) metallacycles, [{C₆H₄-CH₂NMe₂-2}Au{N(Ph)C(O)CHC(O)CH₃}] (4.13), was structurally characterised. A novel and unprecedented dimerisation occurred with [{C₆H₃-CH₂NMe₂-2-OMe5}Au{N(CO₂Et)C(O)CHCN}], that formed an eight-membered metallacycle (4.15) which was characterised by an X-ray diffraction study. The reactions of cis-[PtCl₂(PPh₃)₂] with either cyanoacetic acid or phenoxyacetic acid in the presence of silver(I) oxide are reported in chapter five. The reaction of phenoxyacetic acid with cis-[PtCl₂(PPh₃)₂] was found to form the simple platinum(II) bis(carboxylate) complex, cis-[Pt(O₂CCH₂OPh)₂(PPh₃)₂] (5.9). Conversely, cyanoacetic acid undergoes a novel decarboxylation reaction which results in the formation of the platinum(II) bis(alkyl) complex cis-[Pt(CH₂CN)₂PPh₃)₂] (5.7). The X-ray crystal structures of these two complexes are described. Chapter six discusses the triethylamine-mediated syntheses of four-membered metallalactam complexes. The reactions of cyanoacetylurea [NH₂C(O)NHC(O)CH₂CN] with cis-[PtCl₂(PPh₃)₂], [PtCl₂(COD)], [PtCl₂(dppe)] and [PdCl₂(dppe)] in the presence of triethylamine and details of their characterisation are described. One complex, cis-[Pt{N(C(O)NH₂)C(O)CHCN}(PPh₃)₂] (6.5), was structurally characterised and was found to crystallise as an H-bonded dimer. The reactions of N-cyanoacetylurethane, 2-benzoylacetanilide and acetoacetanilide with cis-[PtCl₂(PPh₃)₂] in the presence of silver(I) oxide are known to give well-characterised four-membered metallalactam complexes. The results of the reactions of these compounds when reacted under similar conditions, but with triethylamine as the base and halide abstractor, are also discussed. Chapter seven describes a selection of four crystal structures. The X-ray structure of the first coordinatively saturated iridaureylene complex, [Ir{N(Me)C(O)NAc}(Cp*)(PPh₃)] (7.4), which is isoelectronic with four-membered metallalactam complexes, is described. The complex was found to adopt a “bar-stool” geometry. The structures of the complexes, [Rh{O₂CC₆H₄-S-2}Cp*]₂ (7.8) and [Ru{O₂CC₆H₄-S-2}(p-cymene)]₂ (7.9) were studied and found to be dimeric compounds. The complexes were found to adopt a pseudo-octahedral geometry, with a three-coordinate, bridging thiolate sulfur. The fourth structural analysis was that of the novel chelation product of cis-[Pt{NCH₂CH₂C(O)}₂(PPh₃)₂] with uranyl nitrate [UO₂(NO₃)₂]. The complex was found to form an eight-membered heterocycle (7.15), with the platinum bis(lactam) complex coordinating to the uranium via the lactam oxygen atoms.