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.