We report a family of organometallic rare-earth complexes with the general formula (COT)M(Cp^ttt) (where (COT)^2- = cyclooctatetraenide, (Cp^ttt)^- = 1,2,4-tri(tert-butyl)cyclopentadienide, M = Y(III), Nd(III), Dy(III) and Er(III)). Similarly to the prototypical Er(III) analog featuring pentamethylcyclopentadienyl ligand (Cp*)^-, (COT)Er(Cp^ttt) behaves as a single-ion magnet. However, the introduction of the sterically demanding (Cp^ttt)^- imposes geometric constraints that lead to a simplified magnetic relaxation behavior compared to the (Cp*)^- containing complexes. Consequently, (COT)Er(Cp^ttt) can be viewed as a model representative of this organometallic single-ion magnet architecture. In addition, we demonstrate that the increased steric profile associated with the (Cp^ttt)^- ligand permits preparation, structural characterization and interrogation of magnetic properties of the early-lanthanide complex, (COT)Nd(Cp^ttt). Such a mononuclear derivative could not be obtained when a (Cp*)^- ligand was employed, a testament to larger ionic radius of this early lanthanide ion.