Two novel mixed-valent manganese phosphonate clusters [(MnMn12O6)-Mn-II-O-III(OH)(6)(O3PC6H11)(10)(py)(6)] (1) and [(Mn4Mn18O12)-Mn-II-O-III(O3PC6H11)(8)(O2CCH3)(22)(H2O)(6)(py)(2)] (2) are reported in this paper. Complex 1 is the first carboxylate-free manganese phosphonate cluster. While compound 2 appears to be the largest manganese cluster thus far that contains phosphonate ligand. Both show slow magnetic relaxation behaviors.