EPR spectroscopy. The EPR spectra were recorded at X-band (ca. 9.4 GHz) mw frequency using a Bruker EMX 300 EPR spectrometer equipped with a 1.8 T electromagnet and a Stinger closed-cycle helium gas cryostat. To prepare samples of 1-Ln in situ, Schlenks containing the parent [Ln(Cp tt )3] with equimolar 2.2.2-cryptand and KC8 were cooled to 240 K and Me-THF was added with stirring for 10 min to give ca. 10 mM solutions of 1-Ln based on full conversion; aliquots of these solutions (sample height ca. 5 cm) were transferred to pre-cooled quartz tubes at -20 °C, rapidly frozen in liquid N2 and flame-sealed under vacuum. Frozen solution samples were stored in liquid nitrogen until measurement. To make up the powder EPR sample of 2, 1-2 cm of a finely ground powder was loaded into a 4 mm diameter quartz tube and sealed under vacuum. Powder spectra were obtained for two rotations at ~30 degrees to one another to identify any features due to polycrystallinity. Field corrections were applied using Bruker strong pitch (g = 2.0028) as a reference, and spectra were baselined before simulation. EPR spectra were simulated using the EasySpin toolbox implemented within Matlab. The simulation model has assumed S = ½ for 1-La ( General synthesis of 1-Ln. THF (5 mL) was added to a pre-cooled (-78 °C) Schlenk flask containing 2-Ln (0.5 mmol), KC8 (0.8 -1 mmol) and 2.2.2-cryptand (0.5 mmol). The reaction mixture was allowed to warm to -35 °C, and all subsequent manipulations were done at this temperature to prevent thermal decomposition. The reaction mixture was stirred for 1 h, allowed to settle, and filtered. The solution was combined with Et2O (5 mL), concentrated to 3 mL, layered with hexane (5 mL) and stored at -78 °C to afford crystals of 1-Ln.