This thesis describes mechanistic investigations of acetate assisted C-H activation, the synthesis of cyclometallated complexes containing nitrogen donor ligands via this method and the applications of cyclometallated complexes in terms of insertion reactions.\ud Chapter one introduces the synthesis, by C-H activation, of cyclometallated complexes containing C,N bidentate ligands of palladium, ruthenium, rhodium and iridium. The introduction also gives an overview on the mechanisms of C-H activation and the applications of C-H activation in catalysis, particularly in direct arylation.\ud Chapter two provides an introduction to the synthesis of arene ruthenium and Cp*M (M = Ir, Rh) half sandwich cyclometallated complexes). The scope of cyclometallation via acetate- assisted C-H activation with different directing groups (pyrazole, pyridine, imines, imidazole, oxazoline and triazole) is discussed. The methodology is extended to six membered rings, non aromatic sp2 and sp3 C-H bonds. Mechanistic investigations using bidentate ligands showed that chelating ligands can prevent the C-H activation process.\ud Chapter three describes a joint computational and experimental study of the cyclometallation reactions of dimethylbenzylamine (DMBA) with [IrC12Cp*]2 using a range of chelating bases. DFT calculations show that facile C-H bond cleavage occurs via 'ambiphilic metal ligand activation' (AMLA) and the ease of C-H activation is governed by the accessibility of the K2-xl base displacement step; thus, more weakly coordinating bases promote C-H activation.\ud Chapter four reports the reactivity of cyclometallated half-sandwich complexes (synthesised in chapter two). Alkynes are shown to insert into the M-C bond. In some cases C-N bond formation occurs to form a heterocycle. The product formed depends on the ease of the reductive elimination step. The relevance of these results to the catalytic synthesis of hetero-and carbocycles is discussed.\ud Throughout the thesis all new compounds are fully characterised spectroscopically and by elemental analysis and several compounds have been structurally characterised by X-ray crystallography
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