Chemistry of the early actinides has undergone a lot of developments in recent years, and due to the need of specialised laboratories to handle these often highly radioactive complexes, computational chemistry has become a powerful aid in understanding the physical properties of these unique systems. This thesis describes a systematic computational study of organoactinide and organometallic model complexes of the form [LAnX]n+ where L is the macrocyclic trans- calixbenzenepyrrolide ligand using density functional theory (DFT) in conjunction with a variety of partition-based methods â Mulliken populations analysis (MPA), Hirshfield population analysis (HPA), natural population analysis (NPA) and the quantum theory of atoms in molecules (QTAIM) â with the aim of probing the electronic structure of the An-X and An-arene bonds as a function of the X ligand. Natural bond orbital (NBO) analysis was also used to study the nature of the An-X bonds, with these results compared to the QTAIM descriptions of covalency and ionicity in the [LAnX]n+ complexes. Analogous transition metal complexes of the form [LMX]n+ (M = Hf, W) have also been studied with the QTAIM and NBO approaches to compare with the actinide-based systems. Nucleus independent chemical shift (NICS) analysis was carried out to probe the extent of aromaticity of the arene rings of the L2- ligand in the closed-shell [LThX]+ complexes as a function of X ligand, and was compared with QTAIM measurements of aromaticity. The MPA also revealed Î´-bonding to the arene rings of the L2- ligand and was compared to the NICS data. Bond energies and bond energy decomposition analysis (EDA) of An-X were further analysed and compared to the QTAIM data. These same analyses were carried out on complexes where the X-type ligand series was extended to include a larger set of first and second-row p-block based ligands. Finally, other, bi-metallic actinide-based complexes including the L2-/4- ligand were studied with the aim of understanding the thermal stabilities of these experimentally-characterised complexes, with analogous model complexes modelled to find potential synthetic targets. The Kohn-Sham molecular orbitals (KS-MOs) of some of these complexes were also analysed to try and find a rationale, based on their electronic structure, for the energetic preference for one binding mode of L-An over another.