Dinuclear Mn(II) complexes having a substitutionally labile {Mn-2(mu-carboxylato)(2)}(2+) core were readily synthesized by using the dinucleating ligand XDK, where H(2)XDK = m-xylenediamine bis(Kemp’s triacid imide). Reaction of Mn(NO3)(2) . 6H(2)O With Na(2)XDK .4H2O resulted in quantitative formation of [Mn-2(XDK)(NO3)(CH3OH)(4)(H2O)(2)]-(NO3) (1), which was characterized by X-ray crystallography (monoclinic, P2(1)/c, a = 11.226(1) Angstrom, b = 13.120-(1) Angstrom, c = 30.467(3) Angstrom, beta = 98.739(8)degrees, V = 4435.2(7) Angstrom(3), Z = 4, and R = 0.034 and R(w) = 0.045 for 4957 independent reflections with I > 3 sigma(I)). The cation in 1 contains two octahedral Mn(II) ions bridged by the two carboxylate groups of XDK (Mn...Mn = 4.8497(7) Angstrom), the terminal positions being occupied by labile solvent molecules and a nitrate anion. Compound 1 proved to be a good precursor for preparing a series of bis(mu-carboxylato)dimanganese(II) complexes with N-donor bidentate terminal ligands. Reaction of 1 with 2,2’-dipyridyl (bpy), 4,4’-dimethyl-2,2’-dipyridyl (4,4’-Me(2)bpy), or 1,10-phenanthroline (phen) afforded [Mn-2(XDK)-L(2)(NO3)(2)(H2O)] (L = bpy (2), 4,4’-Me(2)bpy (3), or phen (4)) in high yields. The structure of 2 . CH2Cl2 was shown by X-ray crystallography to have an asymmetric dinuclear Mn(II) core bridged by XDK with a Mn...Mn distance of 4.557(2) Angstrom. One Mn(II) atom adopts an octahedral geometry while the other has a distorted trigonal bipyramidal environment (monoclinic, P2(1)/c, a = 14.491(2)Angstrom b = 17.954(2) Angstrom c = 22.492(3) Angstrom, beta = 108.787-(9)degrees, V = 5537(1) Angstrom(3), Z = 4, and R = 0.055 and R(w) = 0.058 for 4438 independent reflections with I > 3 sigma(I)). Compounds 2 and 3 are readily oxidized by excess tert-butyl hydroperoxide in methanol to afford (mu-oxo)bis(mu-carboxylato)dimanganese(III) complexes, [Mn-2(mu-O)(XDK)L(2)(NO3)(2)] (L = bpy (5) and 4,4’-Me(2)bpy (6)). The crystal structure of 6 . 2.5CH(3)OH revealed two octahedral Mn(III) ions symmetrically bridged by two carboxylate groups of XDK and an oxo ligand. The metal-metal separation is 3.170(2) Angstrom, and the two 4,4’-Me(2)bpy ligands lie in anti arrangement with respect to the Mn-O-Mn plane (orthorhombic, Pbcn, a = 41.636(9) Angstrom, b = 13.108-(1) Angstrom, c = 22.422(9) Angstrom, V = 12237(2) Angstrom(3), Z = 8, and R = 0.073 and R(w), = 0.085 for 3731 independent reflections with I > 3 sigma(I)). Complexes 5 and 6 could be prepared in low yields by air oxidation of 2 and 3, respectively, a reaction that was readily reversed by treatment with hydrogen peroxide. Reaction of 2-4 with an excess amount of hydrogen peroxide quite slowly evolved dioxygen, whereas the solvento dimanganese(II) compound 1 efficiently disproportionated hydrogen peroxide by first converting to a heterogeneous catalyst. A mononuclear complex, [Mn(HXDK)(2)(H2O)(2)] (7), prepared independently, and MnO2 were isolated from the heterogeneous mixture, and the latter was shown to be the active species for disproportionating hydrogen peroxide.