The syntheses and characterization of a series of binuclear bis(mu-oxo)dimanganese(III,III), -(III,IV), and -(IV,IV) complexes of the general type [(A)4Mn(O)]2(2+/3+/4+), where the symbol A4 represents the four nitrogen atoms from a tetradentate ligand, are described. The ligands (A)4 used are N,N’-bis(2-pyridylmethyl)-N,N’-dimethyl-1,2-ethanediamine (bispicMe2en) and N,N’-bis(2-pyridylmethyl)-N,N’-dimethyl-(-)D-1,2-cyclohexanediamine (bispicMe2-(-)chxn). The syntheses and properties of the (III,III), (III,IV), and (IV,IV), complexes (where (III,III) represents dimanganese(III,III), etc.) of bispicMe2en are described, as are those of the (III,IV) and (IV,IV) complexes of bispicMe2(-)chxn. The (III,IV) complex [(bispicMe2en)Mn(O)]2(ClO4)3.H2O (1) crystallizes in the monoclinic space group P2(1)/c with 4 formula units in a cell of dimensions a = 19.839(4) angstrom, b = 12.468(3) angstrom, c = 19.878(3) angstrom, and beta = 120.657(10)-degrees. The structure refined to a final R-factor of 0.062 based on 2874 reflections. The isomer crystallized is the cis-alpha-species, in which the pyridine nitrogen atoms are trans. The Mn(1)-O bonds are approximately 0.06 angstrom longer than the Mn(2)-O bonds, and the geometry at Mn(1) is significantly axially elongated; hence, the manganese atoms can be distinguished as Mn(1) being Mn(III) and Mn(2) being Mn(IV). The Mn...Mn separation in the dimer is 2.678(2) angstrom. The (III,III) complex [(bispicMe2en)Mn(O)]2(ClO4)2.5H2O (3) crystallizes in space group Aba2 of the orthorhombic system with four dinuclear formula units in a cell of dimensions a = 15.561(3) angstrom, b = 16.703(3) angstrom, and c = 16.430(3) angstrom. The structure has been refined to a final R-factor of 0.0564 based on 2548 observed independent reflections. The isomer is again the cis-alpha-species. There is a crystallographic 2-fold axis passing through the center of the dimer and relating one manganese(III) center to the other. As is expected for a d4 Mn(III) ion, the geometry at manganese is axially elongated, the average Mn-N(axial) bond being approximately 0.12 angstrom longer than the average of the equatorial Mn-N bonds. The Mn...Mn separation in the dimers of 2.699(2) angstrom is comparable to that in related systems. The (III,IV) complex [(bispicMe2(-)chxn)MnO2Mn(bispicMe2(-)chxn)](ClO4)3 (4) crystallizes in the orthorhombic space group P2(1)2(1)2(1) with four dinuclear formula units in a cell of dimensions a = 13.098(3) angstrom, b = 16.297(3) angstrom, and c = 23.273(4) angstrom. The structure has been refined to a final R-factor of 0.0620 based on 3057 observed independent reflections. The isomer is cis-alpha, with roughly isotropic geometry at Mn(2) and axial elongation at Mn(1). Thus atom Mn(1) is the localized d4 Mn(III) site and Mn(2) is the localized d3 Mn(IV) center. The Mn...Mn separation in the dimer is 2.693(2) angstrom. The complexes exhibit antiferromagnetic interactions, the (III,III) complex 3 having 2J = -201 cm-1, the (IV,IV) complexes 2 and 5 2J values of -205 and -211 cm-1, respectively, and the (III,IV) complexes 1 and 4 2J values of -320 and -293 cm-1, respectively.