Tetranuclear cubane-type rare-earth methylidene complexes consisting of four "Cp'LnCH(2)" units, [Cp'Ln(mu(3)-CH2)](4) (4-Ln; Ln = Tm, Lu; Cp' = C5Me4SiMe3), have been obtained for the first time through CH4 elimination from the well-defined polymethyl complexes [Cp'Ln(mu(2)-CH3)(2)](3) (2-Ln) or mixed methyl/methylidene precursors such as [Cp'(3)Ln(3)(mu(2)-Me)(3)(mu(3)-Me)(mu(3)-CH2)] (3-Ln). The reaction of the methylidene complex 4-Lu with benzophenone leads to C=O bond cleavage and C=C bond formation to give the cubane-type oxo complex [Cp'Lu(mu(3)-O)](4) and CH2=CPh2, while the methyl/methylidene complex 3-Tm undergoes sequential methylidene addition to the C=O group and ortho C-H activation of the two phenyl groups of benzophenone to afford the bis(benzo-1,2-diyl)ethoxy-chelated trinuclear complex [Cp'Tm-3(3)(mu(2)-Me)(3){(C6H4)(2)C(O)Me}] (6-Tm).