The use of the anion of 3-methyl-1,3,5-pentanetriol (mpt(3-)) in manganese carboxylate chemistry has afforded the new Mn-12(II/III) duster [(Mn6M6III)-M-II(mpt)(6)(CH3CO2)(12)(py)(6)]center dot 3CH(3)CN (1 center dot 3CH(3)CN). Complex 1 was isolated in moderate yield by the reaction of Mn(CH3CO2)(2)center dot 4H(2)O and H(3)mpt in a 2.6:1 molar ratio in a solvent mixture of acetonitrile and pyridine. The structure of 1 consists of alternating [Mn-2(II)(CH3CO2)(3)(py)](+) and [Mn-2(III)(mu-OR)(2)(CH3CO2)(py)](3+) dimeric units (three of each dimer), linked at each end by two alkoxo and one acetate bridges; the mpt(3-) ligands adopt the eta(2):eta(2):eta(2):mu(4) coordination mode. The overall metal topology of this new Mn-12 wheel resembles a guitar plectrum, or a Reuleaux triangle. Complex 1 displays an unprecedented structural topology, being the first example of a (Mn6Mn6III)-Mn-II wheel constructed from alternating homovalent dimers and the only known Mn-12 loop with the trigonal symmetry of a Reuleaux triangle (all other reported loops were ellipsoids). Variable-temperature, solid-state direct- and alternating-current magnetization studies were carried out on complex 1, revealing the presence of antiferromagnetic exchange interactions between the metal ions in the molecule, which lead to a spin ground-state value S = 0; the exchange coupling parameters J were calculated using density functional theory employing a hybrid B3LYP functional.