| Summary: | The first examples of crystallographically characterizable complexes of Tb, Pr, Gd, and Lu have been isolated, which demonstrate that Ln ions are accessible in soluble molecules for all of the lanthanides except radioactive promethium. The first molecular Tb complexes have been obtained from the reaction of Cp'Ln (Cp' = CHSiMe, Ln = rare earth) with potassium in the presence of 18-crown-6 in EtO at -35 °C under argon: [(18-crown-6)K][Cp'Tb], {[(18-crown-6)K][Cp'Tb]}, and {[K(18-crown-6)](μ-Cp')}{Cp'Tb}. The first complex is analogous to previously isolated Y, Ho, and Er complexes, the second complex shows an isomeric structural form of these Ln complexes, and the third complex shows that [(18-crown-6)K] alone is not the only cation that will stabilize these reactive Ln species, a result that led to further exploration of cation variants. With 2.2.2-cryptand in place of 18-crown-6 in the Cp'Ln/K reaction, a more stable complex of Tb was produced as well as more stable Y, Ho, and Er analogs: [K(2.2.2-cryptand)][Cp'Ln]. Exploration of this 2.2.2-cryptand-based reaction with the remaining lanthanides for which Ln had not been observed in molecular species provided crystalline Pr, Gd, and Lu complexes. These Ln complexes, [K(2.2.2-cryptand)][Cp'Ln] (Ln = Y, Pr, Gd, Tb, Ho, Er, Lu), all have similar UV-vis spectra and exhibit Ln-C(Cp') bond distances that are 0.03 Å longer than those in the Ln precursors, Cp'Ln. These data, as well as density functional theory calculations and EPR spectra, suggest that a 4f5d description of the electron configuration in these Ln ions is more appropriate than 4f.
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