AbstractA strictly heterodinuclear MnIII–Gd complex was prepared from a tripodal ligand and characterized by positive fast‐atom bombardment mass spectrometry, which evidenced replacement of a nitrato counterion by a monoethylcarbonato anion. Variable‐temperature direct current and alternating current studies confirm that the MnIII–Gd magnetic interaction is antiferromagnetic. Fitting of the magnetic susceptibility data yields an extremely weak interaction parameter JMnGd equal to –0.04 cm–1, along with a weak axial zero‐field splitting parameter DMn = 0.1 cm–1. This result confirms that a change in the manganese oxidation state, from MnII to MnIII, corresponds to a radical transformation of the magnetic behavior of Mn–Gd, from a ferromagnetic to an antiferromagnetic interaction. In accordance with the magnetic behavior of CoII–Gd complexes previously studied (high‐spin and low‐spin CoII ions), the observed magnetic behavior change may be explained by the difference in the electronic occupancy of the Mn 3d orbital, which is singly occupied for MnII and vacant for MnIII.