Within this thesis, research related to the synthesis of Pt(II) and Pt(IV) cyclometallated complexes involving both aryl and alkyl C-H activation is discussed. The introduction reviews literature relevant to C-H activation, cyclometallation and reductive elimination, both historic and current. Four pyridine based pro-ligands, all with a 4-fluorophenyl substituent and a varying alkyl substituent, were synthesised in order to study both aryl and alkyl C-H activation. Initial reactions with K2PtCl4 led to the cyclometallation of the phenyl ring via a reversible aryl C-H activation, which then resulted in a variety of Pt(II) complexes by the addition of different co-ligands. Oxidation of these Pt(II) species gave Pt(IV) complexes, which subsequently underwent isomerisation to give the thermodynamic product. For the DMSO derivatives this was shown to occur via five coordinate complexes stabilised by agostic interactions with the alkyl chain and resulted in the first crystallographically characterised oxygen bound DMSO Pt(IV) complexes. Pt(II) and Pt(IV) sp2 cyclometallated complexes, each with an sp3 agostic interaction, were isolated and characterised when using the more sterically demanding tbutyl derivative of the pro-ligand. Subsequent alkyl C-H activation of the tbutyl group was observed via an intra-molecular transcyclometallation reaction to give complexes with an sp3 cyclometallation. From these species a series of complexes with a C(sp2)^N^C(sp3) tridentate ligand were also synthesised. It was also noted that a reversible rollover reaction of the Pt(II) agostic complex occurred in DMSO solvent by exchanging the C(sp2)^N cyclometallation for a C(sp2)^C(sp2) cyclometallation via decyclometallation, rotation about the central C-C bond and subsequent C-H activation. Reductive elimination of a chloride ligand with both sp2 and sp3 cyclometallated carbons occurred to give functionalised phenyl rings and tbutyl groups, respectively. Steric interactions were shown to play a major role in these reactions.