Immediate color changes were observed after the addition of TEMPO • to 1-Ln: a bright orange reaction mixture was observed for Tm and a dark green reaction mixture was observed for Yb. Addition of TEMPO • at -78 °C was found to be crucial to prevent the formation of [Ln(TEMPO)2(µ-TEMPO-κ-O,N)]2, which was identified by XRD studies of crystals obtained following reactions that were performed at room temperature. The Eu 3+ analogue 2-Eu could not be prepared from the reaction of 1-Eu with TEMPO • , with the starting materials identified in the reaction mixture by single crystal XRD; this was attributed to the smaller reduction potential of Eu 2+ (E Ө Eu 3+ → Eu 2+ = -0.35 V) compared with the other Ln 2+ ions investigated herein: E Ө Ln 3+ → Ln 2+ : -1.15 V (Yb), -1.55 V (Sm), -2.3 V (Tm). Scheme 1. Synthesis of 2-Ln; the synthesis of 2-Sm has previously been reported in reference . values; this has frequently been observed for Ln complexes containing {N(Si i Pr3)2}, including complexes from the 1-Ln, 2-Sm and 3-Ln families, and as with previous work we attribute this observation to incomplete combustion resulting from the formation of silicon carbides. The FTIR spectra of 2-Ln and 4-Ln differ due to the presence of TEMPO • radicals in the latter series; the O-N bond stretching frequency in these complexes are all found to be 1642 cm -1 , ca. 60 cm -1 lower than the free radical but comparable to that observed for other Ln complexes containing bound TEMPO • radicals, e.g. [La(hfac)3(TEMPO)2] (hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate; ῦ = 1649 cm -1 ). 9