Magnetic exchange in [Mn2(mu-O)3(tmtacn)2]2+: metal-metal bonding or superexchange?

A computational study of the tris-mu-oxo-bridged manganese dimer [Mn2(mu-O)3(NH3)6]2+ as a model for [Mn2(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 A, and a large antiferromagnetic exchange constant, 2Jab = -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.