^Yttrium and all lanthanides except Ce and Pm have been observed in the oxidation state 0 in bis(1,3,5-tri-t-butylbenzene) complexes, see Cloke, F. Geoffrey N. Zero Oxidation State Compounds of Scandium, Yttrium, and the Lanthanides. Chem. Soc. Rev. 1993, 22: 17–24. doi:10.1039/CS9932200017. and Arnold, Polly L.; Petrukhina, Marina A.; Bochenkov, Vladimir E.; Shabatina, Tatyana I.; Zagorskii, Vyacheslav V.; Cloke. Arene complexation of Sm, Eu, Tm and Yb atoms: a variable temperature spectroscopic investigation. Journal of Organometallic Chemistry. 2003-12-15, 688 (1–2): 49–55. doi:10.1016/j.jorganchem.2003.08.028.
^Chen, Xin; et al. Lanthanides with Unusually Low Oxidation States in the PrB3– and PrB4– Boride Clusters. Inorganic Chemistry. 2019-12-13, 58 (1): 411–418. PMID 30543295. doi:10.1021/acs.inorgchem.8b02572.