A new class of oxo-bridged high-valent hexamanganese (Mn-6) clusters containing a novel {Mn6O8}(6+) core, [Mn-4(IV)-Mn-2(III)(mu-O)(4)(mu(3)-O)(4)(dmb)(6)(O2CR)(2)](4+) (where dmb = 4,4'-dimethyl-2,2'-bipyridine, and RCO2 = 2,6-di(p-tolyl)benzoate ((ArCO2-)-C-Tol) (3) or 2,6-di(4-tert-butylphenyl)benzoate (Ar4-tBuPhCO2-) (4)), was synthesized using sterically hindered m-terphenyl-derived carboxylate ligands. These complexes can be synthesized by oxidizing the Mn-II mononuclear complexes, [Mn(dmb)(2)(OH2)(O2CR)](+) (where RCO2 = (ArCO2-)-C-Tol (1) or Ar4-BuPhCO2- (2)) with (n-Bu4N)MnO4, by direct Mn(II) + Mn(VII) in situ comproportionation reactions, or by ligand substitution on the dinuclear manganese (III,IV) or (IV,IV) complexes, [(Mn-2(mu-O)(2)(dmb)(4))](3+/4+). The compound [(Mn4Mn2III)-Mn-IVM(mu-O)(4)(mu(3)-O)(4)(dmb)(6)((ArCO2)-C-Tol)(2)]-(OTf)(4) [3(OTf)(4)] crystallizes in the monoclinic space group P2(1)/n, with the cell parameters a = 15.447(1) Angstrom, b = 15.077(2) Angstrom, c = 27.703(2) Angstrom, beta = 91.68(2)degrees, V = 6449.3(6) Angstrom(3), and Z = 2. The X-ray structure reveals that there are three different bridging modes for the oxo groups: mu, "pyramidal" mu(3), and "T-shaped" mu(3). Solid-state variable temperature magnetic susceptibility studies suggest that the Mn centers are net antiferromagnetically coupled to yield a diamagnetic S-T = 0 ground spin state with a large number of low-lying, thermally accessible states with S-T > 0. H-1 NMR spectra were recorded for both Mn-6 Clusters and selected resonances assigned. The electronic and redox properties of these complexes along with the effect of the presence of the bulky carboxylate ligands are also described here.