Heterometallic linear tetramers [Mn(5-R-saltmen)Ni(pao)(bpy)(2)](2)-(ClO4)(4) (5-R-saltmen(2)- = N,N'-1,1,2,2-tetramethylethylene bis(5-R-salicylideneiminate); pao(-) = pyridine-2-aldoximate; bpy = 2,2'-bipyridine, R = H, 1; Cl, 2; Br, 3; MeO, 4) have been synthesized and structurally characterized. These compounds exhibit a [Ni-II-NO-Mn-III-(O)(2)-Mn-III-ON-Ni-II] skeleton where -ON- is an oximate bridge between Mn-III and Ni-II ions and -(O)(2)- is a bi-phenolate bridge between Mn-III ions. These tetramers can be seen as oligomeric units of the heterometallic Mn-2(III)-Ni-II chain observed in a family of single-chain magnets (Clerac, R.; Miyasaka, H.; Yamashita, M.; Coulon, C. J Am. Chem. Soc. 2002, 124, 12837. Miyasaka, H.; Clerac, R.; Mizushima, K.; Sugiura, K.; Yamashita, M.; Wernsdorfer, W.; Coulon, C. Inorg. Chem. 2003, 42, 8203.). Magnetic measurements on these tetramers confirm the nature of the magnetic interactions reported for the Mn-2(III)-Ni-II chains: a strong antiferromagnetic Mn-III/Ni-II coupling via the oximate bridge (J(Ni-Mn), ranges from -23.7 to -26.1 K) and a weak ferromagnetic Mn-III/Mn-III coupling through the bi-phenolate bridge (J(Mn-Mn) ranges from +0.4 to +0.9 K). These magnetic interactions lead to tetramers with an S = 2 ground state.