The linear-type heterometallic tetramers, [Mn-2(III)(5-MeOsaltmen)(2)M-2(II)(L)(2)](CF3SO3)(2)center dot 2H(2)O (M-II = Cu, 1a; Ni, 2a), where 5-MeOsaltmen(2-) = N,N '-(1,1,2,2-tetramethylethylene) bis(5-methoxysalicylideneiminate), and H2L = 3-{2-[(2-hydroxy-benzylidene)-amino]-2-methyl-propylimino}-butan-2-one oxime, have been synthesized and characterized from structural and magnetic points of view. These two compounds are isostructural and crystallize in the same monoclinic P2(1)/n space group. The structure has a [M-II-NO-Mn-III-(O)(2)-Mn-III-ON-M-II] skeleton, where -NO- is a linking oximato group derived from the non-symmetrical Schiff-base complex [M-II(L)] and -(O)(2)- is a biphenolato bridge in the out-of-plane [Mn-2(5-MeOsaltmen)(2)](2+) dimer. The solvent-free compounds, 1b and 2b, have also been prepared by drying of the parent compounds, 1a and 2a, respectively, at 100 degrees C under dried nitrogen. After this treatment, the crystallinity is preserved, and 1b and 2b crystallize in a monoclinic P2(1)/c space group without significant changes in their structures in comparison to 1a and 2a. Magnetic measurements on 1a and 1b revealed antiferromagnetic Mn-III center dot center dot center dot Cu-II interactions via the oximato group and weak ferromagnetic Mn-III center dot center dot center dot Mn-III interactions via the biphenolato bridge leading to an S-T = 3 ground state. On the other hand, the diamagnetic nature of the square planar Ni-II center generates an S-T = 4 ground state for 2a and 2b. At low temperature, these solvated (a) and desolvated (b) compounds display single-molecule magnet behavior modulated by their spin ground state.