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The sodium cyclobutadienyl (Cb) reagent [Na2{η4-C4(SiMe3)4}(THF)]2(2) reacts with rare-earth halides [MCl3(THF)x], where M = Y, Dy (x= 3.5) and Lu (x= 3), giving the sandwich coordination polymers [M{η3-C4(SiMe3)4H}{η4-C4(SiMe3)3-κ-(CH2SiMe2}Na]∞(3M, M = Y, Dy, Lu). The coordination environments of the rare-earth metals in 3Mfeature an η4-cyclobutadienyl ligand, with additional coordination by a tuck-in trimethylsilyl group and an η3-cyclobutenyl ligand. The ligand activation is thought to be a consequence of deprotonation of the Cb ligand by 2. The axial nature of the crystal field in 3Mis reflected in the single-molecule magnet properties of 3Dy, which has an anisotropy barrier of 309 cm–1in zero applied magnetic field.