A computational study of the tris-mu-oxo-bridged manganese dimer [Mn-2(mu-O)(3)(NH3)(6)](2+) as a model for [Mn-2(mu-O)3(tmtacn)(2)](2+) (tmtacn = N',N",N"'-trimethyl-1,4,7-triazacyclononane) has been undertaken to investigate the magnetic coupling in this complex. Although the complex has a very short Mn-Mn distance, 2.3 Angstrom, and a large antiferromagnetic exchange constant, 2J(ab) = -780 cm(-1), the calculations reveal that the magnetic coupling is dominated by superexchange via the mu-oxo bridges and that direct Mn-Mn interaction is small.