Hence, attention turned to increasing the magnetic anisotropy. One solution was found in employing lanthanide ions, especially dysprosium(III). Here, the magnetic anisotropy originates from the crystal field splitting of the ground multiplet, resulting in energy barriers up to ca. 2000 K. An alternative approach is the usage of low-coordinate transition metal ions, where spectroscopically certified energy barriers of up to 650 K have been reported. In this respect, four-coordinate cobalt(II) complexes deserve special attention. On the one hand, these complexes are usually chemically quite robust, and on the other hand, large effective energy barriers and slow relaxation of the magnetization at high temperatures have been reported. Four-coordinate cobalt(II) ions possess orbitally non-degenerate spin quartet ground states that are split into two Kramers doublets (KDs) by the ZFS. In spite of the promise of certain mononuclear lanthanide and transition metal complexes as SMMs, true magnetic bistability has been rare. Careful crystal field engineering by tailoring the ligand sphere has allowed sizable coercivities in lanthanides, but this approach is not necessarily robust, when considering that practical application will entail surface immobilization. Developing exchange coupled dinuclear complexes of highly anisotropic ions has proven to be a second viable strategy to engender magnetic bistability in lanthanide complexes, but is less common in transition metal chemistry. One challenge here is that the exchange interactions should not be weaker than the local anisotropy, lest excited spin states are generated, which can serve as intermediate states in Orbach relaxation processes, lowering the effective energy barrier.