Two different coupling strategies were employed to create Er-III single-molecule magnets (SMMs) using high blocking temperature mononuclear precursors. The magnetic properties of three lanthanide-COT complexes, [Er-2(III)(COT '')(3)] (1) (COT '' = 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion) and K-2(THF)(4)[Ln(2)(III)(COT)(4)] (Ln = Gd (2), Er (3); THF = tetrahydrofuran, COT = cyclooctatetraenyl dianion), are reported. Both Er-III complexes behave as SMMs and exhibit magnetic hysteresis at 12 K in solid state. In solution compound 1 exhibits hysteresis up to 14 K. Ac susceptibility data indicates a 100 s blocking temperature of 12.5 and 12.9 K for [Er-2(III)(COT '')(3)] and K-2(THF)(4)[Er-2(III)(COT)(4)], respectively. Both Er-III dimers display enhanced SMM properties over their mononuclear analogues due to their linear structure and strictly axial anisotropy. A 4 K increase in the magnetic blocking temperature of [Er-2(III)(COT '')(3)] over the double-decker analogue is attributed to an additional mechanism of magnetization blocking arising from exchange coupling between Er-III ions.