The reaction between Mn6L12 and Mg6L12 (L = N,N-diethylcarbamate) results in isolation of heteronuclear complexes MnnMg6-nL12. A series was prepared with different doping factors n by varying the Mn/Mg ratio in the crystallization solutions. Single-crystal X-ray diffraction shows that MnMg5L12 is isostructural with MnMg5L12 and Mn6L12. Magnetic susceptibility data on the series MnnMg6-nL12 (n = 1-6) are consistent with antiferromagnetic Mn ... Mn interactions. At low n, the magnetic data demonstrate the formation of magnetically isolated Mn2+ centers. This was confirmed by measurement of the EPR spectrum at a doping factor n = 0.06 in solution, as a powder, and as single crystals. These show hyperfine interactions consistent with isolated Mn2+. The EPR spectrum of Mn0.06Mg5.94L12 exhibits a dominant signal at g(eff) = 4, and a wide series of less intense signals spanning 200-6000 G in the X-band regime. This unusual behavior in a weak-field Mn2+ Complex is attributed to the substantial distortions from cubic ligand field geometry in this system. The g(eff) = 4 signals are attributed to a C-2-symmetric hexacoordinate Mn2+ ion with D > 0.3 cm(-1) and E/D = 0.33. The wide series is assigned to an axial C-4v pentacoordinate Mn2+ site with D = 0.05 cm(-1). Comparison of the g(eff) = 4 signals to the g = 4.1 signals exhibited by the tetramanganese complex in photosystem II belies the fact that they almost certainly arise from different spin systems. In addition, the similarity of the spectrum of MnnMg6-nL12 to mononuclear Mn4+ complexes suggests that considerable care must be exercised in the use of EPR as a fingerprint for the manganese oxidation state, particularly in manganese proteins where molecular composition may not be precisely established.