This thesis describes the synthesis and reactivity of cyclometallated complexes containing nitrogen donor ligands.;Chapter One introduces the general chemistry of cyclometallated complexes containing C,N-bidentate ligands, then describes the mechanism of cyclometallated complexes. This is followed by an overview of the applications of cyclometallated complexes.;Chapter Two provides an introduction to hemilability and reviews the synthesis of cyclopalladated complexes containing C,N,X tridentate ligands (X= N, S, O). The synthesis of new cyclopalladated imines containing oxygen-functionalised tethers is described. Coordination of the oxygen is shown to depend on i) the nature of the oxygen donor, ether, alcohol or phenol. ii) the length of the linker. iii) whether the complexes are neutral or cationic.;Chapter Three establishes the scope of acetate-assisted C-H activation for the synthesis of arene ruthenium and Cp*M (M = Ir, Rh) half-sandwich cyclometallated complexes with nitrogen-donor ligands viz. imine, amine, oxadoline and pyridine and with P(OPh)3. The method can activate sp2 C-H bonds of phenyl, pyrrole and thiophene, and one example of an sp3 C-H bond. The method also allows N-H activation of a pyrrole-imine. Preliminary investigations of the mechanism are also described.;Chapter Four reports the reactivity of the cyclometallated half-sandwich complexes (synthesised in chapter three). Alkenes and CO provide simple substitution products. Alkynes are shown to insert regioselectivity into the M-C bond. In some cases subsequent C-C or C-N bond formation occurs to form carbocyclic or heterocyclic products. With PhC-CH, insertion of two molecules can occur to give novel products. Throughout the thesis X-ray crystallography has been used to verify or identify some of the products (>40 structures).