The reaction of manganese(II) salts with organophosphonic acid [t-BuPO3H2 or cyclopentyl phosphonic acid (C5H9PO3H2)] in the presence of ancillary nitrogen ligands [1,10-phenanthroline (phen) or 2,6-bis(pyrazol-3-Apyridine (dpzpy)], afforded, depending on the stoichiometry of the reactants and the reaction conditions, dinuclear, trinuclear, and tetranuclear compounds, [Mn-2(t-Bu-PO3H)(4)(phen)(2)]center dot 2DMF (1), [Mn-3(C5H9PO3)(2)(phen)(6)]-(ClO4)(2)center dot 7CH(3)OH (2), [Mn-3(t-BuPO3)(2)(dpzpy)(3)](ClO4)(2)center dot H2O (3), [Mn-4(t-BuPO3)(2)(t-BuPO3H)(2)(phen)(6)(H2O)(2)](ClO4)(2) (4), and [Mn-4(C5H9PO3)(2)(phen)(8)(H2O)(2)](ClO4)(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.