Obtaining rational control over the structure and nuclearity of metalloclusters is an ongoing challenge in synthetic Fe-S cluster chemistry. We report a new family of tridentate imidazolin-2-imine ligands L(NIm(R))(3) that can bind [Fe4S4](2+)or [Fe6S6](3+) clusters, depending on the steric profile of the ligand and the reaction stoichiometry. A high-yielding synthetic route to L(NIm(R))(3) ligands (where R is the imidazolyl N substituents) from trianiline and 2-chloroimidazolium precursors is described. For L(NIm(Me))(3) (tris(1,3,5-(3-(N,N-dimethyl-4,5-diphenylimidazolin-2-imino)-phenylmethyl))benzene), metalation with 1 equiv of [PH4P](2)[Fe4S4Cl4] and 3 equiv of NaBPh4 furnishes a mixture of products, but adjusting the stoichiometry to 1.5 equiv of [Ph4P](2)[Fe4S4Cl4] provides (L(NIm(Me))(3))Fe6S6Cl6 in high yield. Formation of an [Fe6S6](3+) cluster using L(NIm(Tol))(3) (tris(1,3,5-(3-(N,N-bis (4-methylphenyl)-4,5-diphenylimidazolin-2-imino)phenylmethyl))-benzene) is not observed; instead, the [Fe4S4](2+) cluster [(L(NIm(Tol))(3))(Fe4S4Cl)][BPh4] is cleanly generated when 1 equiv of [Ph4P](2)[Fe4S4Cl4] is employed. The selectivity for cluster nuclearity is rationalized by the orientation of the imidazolyl rings whereby long N-imidazolyl substituents preclude formation of [Fe6S6](3+) clusters but not [Fe4S4](2+) clusters. Thus, the structure and nuclearity of L(NIm(R))(3)-bound Fe-S clusters may be selectively controlled through rational modification the ligand's substituents.