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Nuclear power currently plays a significant role in today’s balanced energy portfolio. However, with this expansion, comes the need for improved methods to characterise and manage radioactive wastes arising from fission activities throughout the fuel cycle, and the need for fundamental research into all aspects of the nuclear fuel cycle. Whilst the chemistry of actinides is enjoying a renaissance, in particular the study of unusual oxidation states which were previously unobtainable, many fundamental properties of the actinides remain unknown. This thesis explores the coordination chemistry and emission spectroscopy of a selection of actinides (U to Cm) with a view to accessing the unstable f 1 oxidation states of uranium and neptunium. In particular, the role of cation-cation interactions (CCIs) on the stability, redox and spectroscopic properties has been addressed. Work on the luminescence of uranyl(VI) complexes has led to an increased understanding of the non-aqueous solution behaviour of the actinyl ion, in particular by fingerprinting/identifying complexes bearing cation-cation interactions. The uranyl(VI) LMCT emission profiles are observed to significantly red shift when CCIs are present, accompanied by a drastic reduction in the radiative lifetime. These observations have been supported by the increasingly effective diffusion-ordered nuclear magnetic resonance technique. This leads to the conclusion that uranyl(VI) cation-cation interactions unsupported by bridging ligands are unstable in solution. Attempts to isolate stable uranyl(V) complexes of tetraphenylimidodiphosphinate (TPIP) and the fluorinated acacs have proved difficult. The TPIP ligand has shown an unprecedented preference to stabilise NpO2(VI). From these observations, it has been possible to conclude that TPIP has a strong preference for the actinyl(VI) oxidation states. A preliminary study on the minor actinides Am and Cm in the +III oxidation state has taken place and allowed preliminary comparisons between a novel series of bis-trialkylsilyl bipyridyl and phenanthroline ligands, which have been designed in an attempt to study the effect of steric bulk in supporting unusual oxidation states of the actinides. An initial study of these ligands with AnO2(V) and AnO2(VI) (An = U and Np) has also taken place.

The synthesis and characterisation of a series of {AnO₂}ⁿ⁺ complexes containing multidentate ligand environments is reported. Three novel {UO₂}²⁺ complexes (1-3) containing N₃O₂ linear pentadentate ligands have been prepared and crystallographically characterised. NMR spectroscopy has been able to show that 1-3 are stable with respect to ligand exchange, in a range of solvents. The strength of the O=U=O unit has been probed by vibrational spectroscopy and 1-3 exhibit some of the weakest O=U=O ν₁ stretching modes reported (802-805 cm⁻¹). The cyclic voltammetry (CV) of 1-3 in various solvents (0.1 M [Bu₄N][PF₆]) has been performed and indicate the position and reversibility of the {UO₂}²⁺/{UO₂}⁺ redox couple has been found to be subtly dependent on the solvation environment. {UO₂}²⁺ complexes (4-6) have been prepared by subtle modification of reaction conditions using a rigid N₂O₂S linear pentadentate ligand. Characterisation by X-Ray diffraction reveals different monometallic systems, where 4 and 5 are solely bound to the O₂ donors of the ligand and 6 exhibits uranyl binding through all of the donor atoms in the N₂O₂S cavity. ¹H NMR spectroscopy shows 5 exhibits intramolecular rearrangement on the NMR timescale in DCM, but undergoes intermolecular ligand exchange in more coordinating solvents (DMSO, py). Cyclic voltammetry of 5 in DCM (0.1 M [Bu₄N][PF₆]) also indicates that rearrangements and/or ligand exchange processes may occur at rate comparable to that of the CV studies. Complex 6 exhibits stability with respect to ligand exchange or rearrangement in various solvents and shows comparable solvation environment dependency of the {UO₂}²⁺/{UO₂}⁺ redox couple, relative to 1-3. Three monometallic {UO₂}²⁺ complexes (7-9) have been prepared using a rigid tetradentate N₂O₂ ligand. A dimetallic {UO₂}²⁺ complex (10) where two {UO₂}²⁺ are linked by a 4,4'-bipyridine bridge, has been formed by controlled ligand exchange. Vibrational spectroscopy shows the presence of the O=U=O ν₁ stretch in both the Raman and infrared spectra for 7-10, likely to be caused by distortion of the ligand about the UO₂}²⁺ equatorial plane causing a change in dipole for the O=U=O ν₁ stretching mode. A synthetic and spectroscopic study of neptunyl coordination to N₃O₂ linear pentadentate ligands has been made. Adopting a similar method that was successfully employed in the synthesis of 1-3, resulted in the reduction of {NpO₂}²⁺ to {NpO₂}⁺, giving a series of complexes (11-13). Complex 13 has been crystallographically characterised and shows a monometallic {NpO₂}⁺ complex which is bound to all of the atoms in the N₃O₂ cavity.