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The separation of minor actinides (An) such as americium and curium (Am, Cm) from lanthanides (Ln) in spent nuclear fuel can reduce the radiotoxicity of the eventual waste product as well as the required size and environmental impact of any subsequent geological disposal. In addition, separation of these actinides from the lanthanides is essential for a strategy which aims to put the minor actinides back into the fuel cycle through transmutation by neutron bombardment, which would increase fuel efficiency. This work uses Density Functional Theory (DFT) and the Quantum Theory of Atoms in Molecules (QTAIM) to investigate the structure, stabilities and covalency of complexes of the lanthanides and minor actinides with several nitrogen donor ligands which have been developed for the difficult task of AnIII/LnIII separation. A systematic QTAIM study of Ln bond characterisation across the series is reported for one such ligand, bis-triazinyl-pyridine (BTP), confirming the general assumption that bonding in these complexes is ionic in character and largely similar. A small yet significant increase of the charge accumulation in the bonds of the An complexes of BTP was observed, and DFT studies of the An and Ln complexes found a slight energetic preference of the ligand for An complexation, together implying a small electronic contribution to the experimentally observed selectivity of the BTP ligand. A second nitrogen donor ligand, bis-triazinyl-phenanthroline (BTPhen) was studied, finding slightly higher measures of covalency in the metal-ligand bonds and a greatly improved energetic preference for An complexation. The effects of the addition of electron-directing groups to this ligand were investigated, finding little difference in the measures of covalency for these modified ligands. Several other nitrogen donor and mixed nitrogen/oxygen donor ligands were studied, including a novel sandwich complex, ultimately demonstrating a tentative correlation between enhanced covalency and stability.