The unusual formation of planar trimethylenemethane (TMM) dianion complexes of lanthanide metallocenes, [(C5Me5)(2)Ln](2)[mu-n(3):n(3)-C(CH2)(3)] (Ln = Sm, 1; La, 2; Pr, 3; Nd, 4; Y, 5) has been examined by synthesizing examples with metals from La to Y to examine the effects of radial size on structure and to provide closed shell examples for direct comparison with density functional theory (DFT) calculations. Synthetic routes to 1-4 have been expanded from the [(C5Me5)(2)Ln][(mu-Ph)(2)BPh2]/neopentyl lithium reaction involving beta-methyl elimination to a [(C5Me5)(2)Ln][(mu-Ph)(2)BPh2]/isobutyl lithium route. The synthetic pathways are sensitive to metal radius. To obtain 5, the methylallyl complex, (C5Me5)(2)Y[CH2C(Me)CH2], 6, was synthesized and treated with [(C5Me5)(2)YH](x). In the Lu case, the neopentyl complex [(C5Me5)(2)LuCH2C- (CH3)(3)](x), 7, was isolated instead of the TMM product. X-ray crystallography showed that the metrical parameters of the planar TMM dianions in each complex are similar. The structural data have been compared with DFT calculations on the closed-shell lanthanum and lutetium complexes that suggest only limited covalent interactions with the lanthanide ion. Benzophenone (2 equiv) reacts with 1 to expand the original four-carbon TMM skeleton to a dianionic bis(alkoxide) ligand containing a symmetrically substituted C=CH2 moiety in [(C5Me5)(2)Sm](2)[mu-(OCPh2CH2)(2)C=CH2], 8. In this reaction, the TMM complex reacts as a bifunctional bisallylic reagent that generates an organic framework containing a central vinyl group.