“…[26] Structurally characterized mixed-valent lanthanoid metal organometallic compounds are very rare,s pecifically,p entaalkylcyclopentadienyl Sm (II/III) [27] and Yb (II/III) [28] complexes, for example,[ Cp* 2 Yb III (m-F)Yb II Cp* 2 ]; [28c] an indenyl [29] and two phenyl [30] Yb (II/III) complexes.M ixed valent lanthanoid metal(II/III) compounds supported by O, S, and Se donor ligands have been serendipitously isolated, including two bulky aryloxide Ln (II/III) (Ln = Yb, [31a] Eu [31b] )c omplexes, at hiophenyl Eu (II/II) , [32] and two benzeneselenolate Ln (II/III) (Ln = Sm, Yb) complexes. [33] Of N-donor ligands,abulky pyrazolate Yb (II/III) , [34] ac arbazolate Sm (II/III) , [35] and ac rossbridged cyclam supported Yb (II/III) [36] complexes have been isolated. Thei solation of rare mixed-valent lanthnoid complexes 1, 2,a nd the previously reported [(thf) 2 Sm II -(N 4 Et 8 )Sm III (m-Cl) 2 Li(thf) 2 ] [10] have demonstrated that the tetraanionic calix [4]pyrrolide has the ability to stabilize mixed-valent rare-earth metals.T he macrocyclic ligand, constraining both Ln II and Ln III atoms in the cavity to ensure au nique Ln II ···Ln III close contact, and sandwiching each metal with two h 5 bonds with ar einforcement of the coordination by two additional h 1 bonds from nitrogen atoms, may be an ideal system to accommodate mixed valent metal atoms.…”