The preparation and characterization of [Mn-IV(salpn)(mu-OH)](2)(CF3SO3)(2), 1H(2)(CF3SO3)(2), the first bis-(hydroxo)-bridged manganese(IV) dimer, is reported. Along with the previously reported complexes [Mn-IV(salpn)(mu-O)](2), 1, and [Mn-IV(salpn)](2)(mu-O)(mu-OH)(CF3SO3), 1H(CF3SO3), this provides the first series of ore-bridged manganese complexes which are isolated in three different protonation states of the bridges without a change in the other ligands or the manganese oxidation state. The effects of oxo bridge protonations on the structure and magnetism of these Mn(IV) dimers are studied using this series of complexes. Each successive protonation of the oxo bridges results in a 2200 cm(-1) decrease in the energy of the lowest energy (presumably, phenolate-to-manganese) charge transfer transition, a lengthening of the Mn-Mn distance, and a dramatic decrease in the antiferromagnetic coupling, without a significant change in the manganese X-ray absorption edge energies. The edge energies an all within 0.3 eV of each other, at ca. 6553 eV. The extended X-ray absorption fine structure spectra indicate that the Mn-Mn distance increases from 2.73 Angstrom for 1 to 2.83 and 2.93 Angstrom with successive protonations. Temperature-dependent magnetic susceptibility measurements show that the antiferromagnetic coupling of 1 decreases from J = -92 cm(-1) to J = -48 cm(-1) upon the first protonation, and to J = -6 cm(-1) upon the second protonation. Such a low coupling as found for 1H(2)(CF3SO3)(2) is unprecedented for Mn(IV) dimers. Understanding the effects of protonation on the electronic and geometric structure of high-valent Mn-2(IV)(mu-O)(2) units is important, as this core has been proposed to exist in the oxygen-evolving complex (OEC) of photosystem II. The possible effects of protonation state changes on S-state advancement and the proposed different configurations of S-1 and S-2 are discussed.