With the exception of systems containing Cu2+, complex metal oxides containing paramagnetic transition metal cations in square-planar coordination are rare. However, by either introducing chalcogenide, pnictide, or halide anions to form mixed-anion systems or by utilizing low-temperature topochemical reduction via anion deintercalation, an extensive range of phases containing square-planar MO4 units can be prepared. The crystal chemistry of a series of transition-metal A(2)MO(2)X(2) oxyhalide and A(2)MO(2)B(2)X(2) oxychalcogenide and oxypnictide phases is reviewed along with how their magnetic behavior changes as a function of the transition-metal and electron count. As a contrast, these mixed-anion phases are then compared to a series of topochemically reduced, metastable transition-metal oxides that also contain square-planar coordinated transition metals, to more fully illustrate the magnetic properties of extended frameworks of square-planar MO4 units.