We previously reported how the synthesis of [Mn(III)12Mn(II)7(μ4-O)8(μ3-η(1)-N3)8(HL(1))12(MeCN)6]Cl2·10MeOH·MeCN (1), which has a Mn19 core corresponding to two supertetrahedral {Mn(II)4Mn(III)6} units sharing a common Mn(II) vertex, can be modified such that the central octacoordinate Mn(II) ion can be replaced by metal ions more likely to favor this coordination geometry such as Dy(III) as exemplified in the compound [Mn(III)12Mn(II)6Dy(III)(μ4-O)8(μ3-Cl)6.5(μ3-N3)1.5(HL)12(MeOH)6]Cl3·25MeOH (2). Here, we report a systematic survey of the effects of incorporating various diamagnetic metal ions M(n+) into this central position. We chose diamagnetic ions with electron configurations with fully occupied or completely empty frontier orbitals in order to gauge the effect on the overall magnetic behavior. The syntheses, structures, and magnetic properties of the heterometallic aggregates [Mn(III)12Mn(II)6Sr(II)(μ4-O)8(μ3-η(1)-N3)7.5(μ3-η(1)-Cl)0.5(HL(1))12(MeCN)6]Cl2·15MeOH (3), [Mn(III)12Mn(II)6Y(III)(μ4-O)8(μ3-η(1)-N3)8(HL(1))12(MeCN)6](NO3)3·11MeOH (4), [Mn(III)12Mn(II)6Cd(II)(μ4-O)8(μ3-η(1)-N3)6.8(μ3-η(1)-Cl)1.2(HL(1))12(MeCN)6](CdCl4)0.25Cl1.5·14.5MeOH (5), and [Mn(III)12Mn(II)6Lu(III)(μ4-O)8(μ3-η(1)-N3)6.5(μ3-η(1)-Cl)1.5(HL(2))12(MeCN)6]Cl3·3H2O·7MeOH·MeCN (6) (H3L(1) = 2,6-bis(hydroxymethyl)-4-methylphenol, H3L(2) = 2,6-bis(hydroxymethyl)-4-fluorophenol) are reported. The aggregates were prepared in one-pot self-assembly reactions of H3L(1) (or H3L(2)), MnCl2·4H2O or Mn(NO3)2·4H2O, NaOAc·3H2O or Et3N, and NaN3 in the presence of the appropriate diamagnetic metal salt in MeCN/MeOH mixtures. Compounds 3-6 crystallize isotypically to 1 in the trigonal space group R3̅ with Z = 3. The effects on the magnetic properties were investigated, paying attention to the presence of any weak coupling mediated by the diamagnetic cations between the two {Mn(II)3Mn(III)6} S = 39/2 subunits. In the Cd(2+) compound 5, the two {Mn(II)3Mn(III)6} units are magnetically isolated. In 3, 4, and 6, the diamagnetic Sr(2+), Y(3+), and Lu(3+) cations mediate weak antiferromagnetic interactions between the two {Mn(II)3Mn(III)6} subunits. DFT calculations show that the inter-{Mn(II)3Mn(III)6} interactions in the Mn18M systems are attributable to the electronic structure of the central diamagnetic cation, with systems containing trivalent central cations showing stronger antiferromagnetic interactions than those with isoelectronic divalent cations.