(2-Sm), 3 H6 (2-Tm) and 2 F5/2 (2-Yb). Below 10 K for 2-Tm, there is a more rapid decrease in χT, suggesting a split ground pseudo-doublet, which is non-degenerate as Tm 3+ is a non-Kramers ion. Examination of the reduced magnetization plots (magnetization vs. field/temperature) provides information on the ground state (Figure ). For all 2-Ln, the isothermal reduced magnetization curves overlay, indicating a wellisolated ground state. The room temperature χT values for 4-Ln are expected to be the sum of the Ln 3+ contribution and a spin ½ (0.375 cm 3 K mol -1 ), as Ln-radical exchange coupling is expected to be weak and hence negligible at room temperature. As there is a change in formal charge on the coordinating TEMPO •/-oxygen atom and there are small changes in geometry of the [Ln(N † † )(TEMPO)] 0/+ species, the Ln 3+ CF is expected to be modified from 2-Ln to 4-Ln, so the difference in χT (ΔχT) may differ by more or less than the radical contribution even if the effect of exchange is minor. Smaller errors in the diamagnetic corrections and the residual field in the instrument, as well as any remaining co-crystallized solvent, sample decomposition or impurities may also affect ΔχT. For 4-Sm, the room temperature χT is slightly below what is expected for Sm 3+ + radical, and the ΔχT between 2-Sm and 4-Sm is +0.19 cm 3 K mol -1 at 300 K (Table ), and approximately constant from 50-300 K (Figure ). Metal-radical coupling takes effect below 50 K, where χT rapidly decreases as exchange states are depopulated (Figure ). For 4-Tm, the room temperature χT value of 6.70 cm 3 K mol -1 is within the expected range for Tm 3+ plus radical (Table ). The difference between 4-Tm and 2-Tm is non-linear but centered around +0.375 cm 3 K mol -1 between 100-300 K Figure ), with a value of +0.29 cm 3 K mol -1 at room temperature, again supporting that the CF is modified going from 2-Tm to 4-Tm. Here, the exchange coupling is evident below 100 K, so is likely larger than in 4-Sm consistent with a larger dipolar interaction with the more magnetic Tm 3+ ion. The non-overlapping reduced magnetization indicates low energy exchange states, but the data could not be fit with an Ising-like coupling between the effective spin ½ of the pseudo-doublet and the radical S = ½ (Figure ).