The reaction of [Mn3O(O2CMe)(6)(py)(3)](ClO4) (1; 3Mn(III)) with [Mn10O4(OH)(2)(O2CMe)(8)(hmp)(8)](ClO4)(4)(2; 10Mn(III)) in MeCN affords the new mixed-valent complex [Mn21O14(OH)(2)(O2CMe)(16)(hmp)(8)(pic)(2)(py)(H2O)](ClO4)(4) (3; 3Mn(II)-18Mn(III); hmp(-) is the anion of 2-(hydroxymethyl)pyridine), with an average Mn oxidation state of +2.85. Complex 3(.)7MeCN crystallizes in the triclinic space group F (1) over bar. The structure consists of a low symmetry [Mn-21(mu(4)-O)(4)(mu(3)-O)(12)(mu-O)(16)] core, with peripheral ligation provided by 16 MeCO2-, 8 hmp(-), and 2 pic(-) groups and one molecule each of water and pyridine. The magnetic properties of 3 were investigated by both dc and ac magnetic susceptibility measurements. Fitting of dc magnetization data collected in the 0.1-0.8 T and 1.8-4.0 K ranges gave S = 17/2, D approximate to 0.086 cm(-1), and g approximate to 1.8, where S is the molecular spin of the Mn-21 complex and D is the axial zero-field splitting parameter. ac susceptibility studies in the 10-997 Hz frequency range reveal the presence of a frequency-dependent out-of-phase ac magnetic susceptibility (chi(M)") signal consistent with slow magnetization relaxation rates. Fitting of dc magnetization decay versus time data to the Arrhenius equation gave a value of the effective barrier to relaxation (U-eff) of 13.2 K. Magnetization versus applied dc field sweeps exhibited hysteresis. Thus, complex 3 is a new member of the small but growing family of single-molecule magnets.