Trinuclear molecular complexes of europium (II)and ytterbium(II) [Ln3{(Ph2SiO)2O}3(THF)6],1-Ln3L3(Ln = Euand Yb), supported by the dianionic tetraphenyl disiloxanediolateligand, were synthesized via protonolysis of the [Ln{N-(SiMe3)2}2(THF)2] complexes. In contrast, the reaction of[Sm{N(SiMe3)2}2(THF)2] with the (Ph2SiOH)2O ligand led tothe isolation of the mixed-valent Sm(II)/Sm(III) complex[Sm3{(Ph2SiO)2O}3{N(SiMe3)2}(THF)4],2-Sm3L3, which wascrystallographically characterized. The Eu(II) complex1-Eu3L3displays weak ferromagnetic coupling between the Eu(II) metalcenters (J= 0.1035 cm-1). The addition of 3 equiv of (Ph2SiOK)2Oto1-Eu3L3resulted in the formation of the polynuclear Eu(II)dimer of dimers [K4Eu2{(Ph2SiO)2O}4(Et2O)2]2,3-Eu2L4. Com-plexes1-Ln3L3(Ln = Eu and Yb) are stable in solution at room temperature, while3-Eu2L4shows higher reactivity and rapidlydecomposes to give the mixed-valent Eu(II)/Eu(III) species [K3Eu2{(Ph2SiO)2O}4],4-Eu2L4. Complex1-Yb3L3affects the slowreductive disproportionation of carbon dioxide, but1-Eu3L3does not display any reactivity toward CO2. However, the presence ofone additional (Ph2SiO-)2O per Eu(II) metal center in3-Eu2L4increases dramatically the reductive ability of the Eu(II) metalcenters, affording thefirst example of carbon dioxide activation by an isolated divalent europium complex. The reduction of CO2by3-Eu2L4is immediate, and carbonate is formed selectively after the addition of a stoichiometric amount of CO2