General methods and materials. All manipulations were conducted under argon with the strict exclusion of oxygen and water by using Schlenk line and glove box techniques. Solvents were dried by refluxing over Na/K alloy (diethyl ether) or potassium (hexane), and stored over a potassium mirror, then degassed before use. To make up solution samples for NMR, EPR and UV-Vis-NIR spectroscopy, hexane, toluene, THF, Me-THF, C6D6 and C4D8O were dried by refluxing over K, and were vacuum transferred and degassed by three freeze-pump-thaw cycles before use. Elemental )x] (M = La, Ce, Pr, x = 4; M = Nd, x = 3.5) and [UI3(THF)4] were prepared according to literature procedures. ATR-IR spectra were recorded as microcrystalline powders using a Bruker Alpha spectrometer with Platinum-ATR module. UV-Vis-NIR spectroscopy was performed on samples in Youngs tap-appended 10 mm path length quartz cuvettes on an Agilent Technologies Cary