Here we describe the synthesis and magnetic characterization of a family of 2D polymetallic oxalate-bridged polymeric networks with general formula [M^II(H₂O)₂]₃[M^III(ox)₃]₂(18-crown-6)₂ (M^III = Cr, Fe; M^II = Mn, Fe, Co, Ni; 18-crown-6 = C₁₂H₂₄O₆). Depending on the nature of the trivalent metal ion, they exhibit ferro- (Cr³⁺) or ferrimagnetic (Fe³⁺) ordering in the 3.6-20 K interval. In contrast with most of the oxalate-bridged CPs reported so far, these complexes do not need any additional templating cation for their assembly and represent the first series of oxalate-based polymeric networks which can be considered intrinsically neutral. As previously observed for other crown ether containing oxalate-based coordination polymers, these compounds are soluble in water, whereas they remain nonsoluble in other organic solvents. Furthermore, when these molecular precursors are subjected to a thermally controlled decomposition process, pure phases of mixed oxides with spinel-like structures can be conveniently generated. Among the resulting materials, the (Mn,Co,Fe)₃O₄ derivative is particularly remarkable, since it behaves as a magnet at room temperature. Finally, taking advantage of the solubility of these molecular precursors, this room-temperature magnetic oxide has been successfully nanostructured onto a Si(110) substrate via the lithographically controlled wetting (LCW) technique.