Computational modelling was conducted in conjunction with experimental collaborators in the Davies group in Leicester1 into the mechanism of C-H functionalisation at Rh, Ir and Ru complexes. In Chapter 3 C-H activation by [Cp*IrCl2]2 and [Cp*RhCl2]2 dimers was investigated, and an intramolecular acetate-assisted mechanism was characterised. Experimental trends in reactivity were rationalised as reactions with Rh operating under thermodynamic control, while Ir complexes operated under kinetic control due to both less stable C-H activation transition states and more stable cyclometallated products. In order to obtain agreement between experiment and computational results, solvation and dispersion effects had to be accounted for and diffuse basis functions were required. Chapter 4 describes computational studies on functionalisation of rhodacycle complexes formed following C-H activation with alkynes to yield 6-membered ring pyridinium and isoquinolinium products. These proceed via a rate-determining migratory insertion followed by reductive coupling. Experimental results also revealed the possibility of 5-membered ring indenylamine formation and this was computed to occur via acetate-association, imine insertion and protonation by acetic acid (Chapter 5). Selectivity between 5- and 6-membered ring formation was found to be determined by the relative barriers of C-C reductive coupling and C-N bond forming imine insertion steps. The related functionalisation of iridacycles formed via C-H activation was computed to have a selectivity determining migratory insertion and rate determining reductive coupling, while the form of the final heterocyclic Ir complex obtained was under thermodynamic control (Chapter 6). In Chapter 7, Ru-catalysed direct C-H alkylation was investigated computationally. To model these reactions in non-polar aromatic solvents, optimisation was required to be performed in solvent. The most stable reaction profile characterised involved initial C-H activation, followed by an SN2 C-I bond cleavage at an arene s-(C-H) complex with a k2 acetate ligand. C-C reductive coupling then completes the cycle.