The reaction of manganese(II) salts with organophosphonic acid [t-BuPO(3)H(2) or cyclopentyl phosphonic acid (C(5)H(9)PO(3)H(2))] in the presence of ancillary nitrogen ligands [1,10-phenanthroline (phen) or 2,6-bis(pyrazol-3-yl)pyridine (dpzpy)], afforded, depending on the stoichiometry of the reactants and the reaction conditions, dinuclear, trinuclear, and tetranuclear compounds, [Mn(2)(t-BuPO(3)H)(4)(phen)(2)]·2DMF (1), [Mn(3)(C(5)H(9)PO(3))(2)(phen)(6)](ClO(4))(2)·7CH(3)OH (2), [Mn(3)(t-BuPO(3))(2)(dpzpy)(3)](ClO(4))(2)·H(2)O (3), [Mn(4)(t-BuPO(3))(2)(t-BuPO(3)H)(2)(phen)(6)(H(2)O)(2)](ClO(4))(2) (4), and [Mn(4)(C(5)H(9)PO(3))(2)(phen)(8)(H(2)O)(2)](ClO(4))(4) (5). Magnetic studies on 1, 2, and 4 reveal that the phosphonate bridges mediate weak antiferromagnetic interactions between the Mn(II) ions have also been carried out.