We report a family of organometallic rare-earth complexes with the general formula (COT)M(Cpttt) (where (COT)2– = cyclooctatetraenide, (Cpttt)– = 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(Cpttt) behaves as a single-ion magnet. However, the introduction of the sterically demanding (Cpttt)– imposes geometric constraints that lead to a streamlined magnetic relaxation behavior compared to the (Cp*)– containing complexes. Consequently, (COT)Er(Cpttt) 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 (Cpttt)– ligand permits preparation, structural characterization and interrogation of magnetic properties of the early-lanthanide complex, (COT)Nd(Cpttt). 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.