The work presented in this thesis discusses the formation and behaviour of a variety of different ArForm rare-earth complexes using five ArForm ligands of varied functionalities (N,N'-bis(Aryl)Formamidine: Aryl = 2,6-difluorophenyl (DFFormH), 2,6-diisopropylphenyl (DippFormH), 4-methyl phenyl (p-TolFormH), and 2-trifluoromethylphenyl). In addition the chemistry of rare-earth 3,5-dimethylpyrazolate (Me2pz) complexes is also discussed. The majority of divalent rare-earth ArForm complexes known have been synthesised by redox transmetallation protolysis (RTP) protocols or salt metathesis in THF. Chapter two examines two different synthetic approaches to form divalent rare-earth N,N'-bis(2,6-difluorophenyl)formamidinate DFForm complexes. Initially complexes were generated by the RTP method utilising three different solvent media (THF, PhMe, and Et2O), where the oxidation state of the resulting complex was dependent on the solvent used. Additionally treatment of DFFormH with Ln0 metals in CH3CN proved to be an effective synthetic route to divalent DFForm complexes for both Eu and Yb. However, when the direct metal synthetic method was extrapolated to the bulky DippForm ligand system, results were varied. Although the reactivity of divalent [Sm(DippForm)2(thf)2] has been studied under a variety of conditions (Scheme 1.6), currently the isostructural ytterbium analogue ([Yb(DippForm)2(thf)2]), has only been treated with halogenating oxidants.[58f] Chapter Three investigates the ability for the DippForm ligands to stabilise metal ketyl complexes generated by treating [Yb(DippForm)2(thf)2] with a variety of different aromatic ketones and 1,2-diketones. Through a single electron redox process, a variety of ketyl complexes was generated and showed interesting structures and reactivity.Although divalent and trivalent ArForm complexes are known, tetravalent species have remained an unexplored area. Chapter Four discusses the synthesis and oxidation of five novel cerium(III) ArForm complexes by trityl chloride, leading to transient cerium(IV) species that were prone to rapid decomposition. A cerium(IV) ArForm complex was successfully generated by an alternative method, namely protolysis reactions between ArFormH and tetravalent cerium silylamides. Some of the work presented in this Chapter has been recently published and these publications are in Appendix A and Appendix B.In the final chapter on ArForm ligands, Chapter Five, the coordination of N,N'-bis(2-trifluoromethylphenyl)formamidinate to rare-earth ions is discussed. It was initially hypothesised that the CF3 group would coordinate to the rare-earth metal centre and then undergo C-F activation to produce rare-earth heteroleptic fluoride complexes. However, surprisingly, the CF3 group underwent complete de-fluorination, producing inorganic fluorides and also poly(trifluoromethylphenyl)amidines. This unexpected result is discussed in terms of the conditions of activation along with the role of the rare-earth element. Chapter Six ventures away from the chemistry of rare-earth formamidinates and discusses the chemistry of rare-earth 3,5-dimethylpyrazolate complexes in terms of structures and their peculiar reactivity, where it appears that these complexes have a high affinity to form oxide cages. Part of this work has been recently published and is presented in Appendix C. Across these Chapters the versatility of rare-earth formamidinate complexes has been demonstrated, with new binding modes to rare-earths identified, new synthetic methods determined, different types of reactivity investigated, and the results have opened the doors to much exciting new chemistry.