The oxidation of Mn-II carboxylates by (NBu4)Cr2O7 in the presence of different phosphonic acids and chelating ligands results in six Cr-III-doped tetranuclear manganese clusters formulated [Mn3CrO2(O2CCH3)(4)(O3PC5H4N)(2)(bPY)(2)] (1), [Mn3CrO2(O2CCH3)(4)(O3PC5H4N)(2)(phen)(2)] (2), [Mn3CrO2(O2CPh)(4)(O3PC5H4NO)(2)(phen)(2)] (3), [Mn3CrO2(O2CPh)(4)(O3PC6H11)(2)(bPY)(2)] (4), [Mn3CrO2(O2CPh)(4)(O3PC6H11)(2)(phen)(2)] (5), and [Mn3CrO2(O2CCH3)(4)(O3PC6H11)(2)(bpy)(2)] (6). Single-crystal X-ray analyses reveal that all the compounds contain similar [M4O2](8+) cores with the four metal sites arranged in planar topologies. The metal ions within the core are bridged by both carboxylate and phosphonate ligands. Tempe ratu re-dependent magnetic measurements show that in all cases dominant antiferromagnetic interactions are propagated between the metal centers. The ac magnetic measurements on compounds 5 and 6 reveal that both the in-phase and the out-of-phase signals are frequency dependent, characteristic of single-molecule magnet behaviors.