Chapter 1 reviews the literature concerning trimethylenemethane and oxodimethylenemethane metal complexes and their r?le in organic synthesis. The preparation and characterisation of ?4-trimethylenemethane metal complexes of iridium, rhodium, osmium and ruthenium are presented in Chapter 2, together with the molecular structure of three of these complexes which establishes the presence of the ?4-trimethylenemethane ligand. The room temperature 1H n.m.r. spectra show the expected features for a static structure and no sign of rotation of the trimethylenemethane ligand was observed at higher temperatures. Preliminary investigation into the reactivity of these trimethylenemethane complexes is detailed in Chapter 3. A series of cationic ?4-trimethylenemethane complexes are prepared by the reaction of [Ir{?4-C(CH2)3}C1 (CO)(PPh3)] with silver hexafluorophosphate in the presence of a donor ligand such as carbon monoxide or ethene. The reactions of these cationic complexes with neutral and anionic nucleophiles are investigated and show that displacement of other donor ligands or attack on co-ordinated carbonyl and ethene ligands occurs, rather than attack at, or displacement of the trimethylenemethane ligand. Chapter 4 describes the reaction of 3-trimethylsilyl-2-(methyl- sulphonyloxymethyl) prop-l-ene with d10-metal complexes of platinum, palladium and nickel. The isolation of ?3-2-trimethylsilylmethylallyl palladium and platinum complexes are of interest since they are postulated intermediates implicated in the generation of the catalytic species [Pd (?3-trimethylenemethane) (PPh3)2]. The d10 -nickel complex [Ni{P(OEt)3}4] catalyses the cycloaddition of trimethylenemethane to electron-deficient alkenes and imines, the latter reaction affords a high yield synthesis of 4-methylenepyrrolidines. The final Chapter describes the reactions of 3-chloro-l-(trimethyl-silyl)propan-2-one and 3-chloro-2-(trimethylsiloxy)prop-l-ene with low-valent metal complexes to afford oxodimethylenemethane complexes of platinum, iridium, and osmium. The molecular structure of two of these complexes establishes the presence of an ?3-oxodimethylenemethane ligand. The silylenol ether, 3-chloro-2-(trimethylsiloxy)prop-l-ene was readily prepared from 3-chloro-l-(trimethylsilyl)propan-2-one, the isomerisation being catalysed by a number of low-valent metal complexes.