More concentrated solutions were found to be more stable over time as the 1 H NMR spectrum of 2 in C 6 D 6 (C = 30-50 mM) revealed no significant formation of thermolysis products over several days. The thermal stability of Lu II complexes appears to be highly dependent on the ligand environment: at room temperature and in hydrocarbon solvents, [K(crypt)][LuCp' 3 ] features a half-life of ca. 20 min, whereas [K(crypt)][Lu(OAr*) 3 ] slowly decomposes over the course of several days. An exceptional thermal stability was reported for [Lu(Cp i Pr5 ) 2 ], synthesized upon extensive heating at elevated temperatures (160-180 °C). 17d